diff --git a/README.md b/README.md
index 4ece3ae..8ef4964 100644
--- a/README.md
+++ b/README.md
@@ -88,6 +88,11 @@ jobs:
     # Default: false
     error-on-no-successful-workflow: ""
 
+    # Fallback SHA to use if no successful workflow run is found. This can be useful in scenarios where you need a specific commit as a reference for comparison, especially in newly set up repositories or those with sparse workflow runs.
+    #
+    # Default: ""
+    fallback-sha: ""
+
     # The type of event to check for the last successful commit corresponding to that workflow-id, e.g. push, pull_request, release etc.
     #
     # Default: push
diff --git a/action.yml b/action.yml
index b6690d4..1bfdd97 100644
--- a/action.yml
+++ b/action.yml
@@ -11,6 +11,10 @@ inputs:
   error-on-no-successful-workflow:
     description: "By default, if no successful workflow is found on the main branch to determine the SHA, we will log a warning and use HEAD~1. Enable this option to error and exit instead."
     default: "false"
+  fallback-sha:
+    description: "Fallback SHA to use if no successful workflow run is found."
+    required: false
+    default: ""
   last-successful-event:
     description: "The type of event to check for the last successful commit corresponding to that workflow-id, e.g. push, pull_request, release etc"
     default: "push"
diff --git a/dist/index.js b/dist/index.js
index 55d2d3b..2f46a59 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -37867,6 +37867,7 @@ const errorOnNoSuccessfulWorkflow = process.argv[4];
 const lastSuccessfulEvent = process.argv[5];
 const workingDirectory = process.argv[6];
 const workflowId = process.argv[7];
+const fallbackSHA = process.argv[8];
 const defaultWorkingDirectory = ".";
 const ProxifiedClient = action_1.Octokit.plugin(proxyPlugin);
 let BASE_SHA;
@@ -37913,18 +37914,24 @@ let BASE_SHA;
             else {
                 process.stdout.write("\n");
                 process.stdout.write(`WARNING: Unable to find a successful workflow run on 'origin/${mainBranchName}', or the latest successful workflow was connected to a commit which no longer exists on that branch (e.g. if that branch was rebased)\n`);
-                process.stdout.write(`We are therefore defaulting to use HEAD~1 on 'origin/${mainBranchName}'\n`);
-                process.stdout.write("\n");
-                process.stdout.write(`NOTE: You can instead make this a hard error by setting 'error-on-no-successful-workflow' on the action in your workflow.\n`);
-                process.stdout.write("\n");
-                const commitCountOutput = (0, child_process_1.spawnSync)("git", ["rev-list", "--count", `origin/${mainBranchName}`], { encoding: "utf-8" }).stdout;
-                const commitCount = parseInt(stripNewLineEndings(commitCountOutput), 10);
-                const LAST_COMMIT_CMD = `origin/${mainBranchName}${commitCount > 1 ? "~1" : ""}`;
-                const baseRes = (0, child_process_1.spawnSync)("git", ["rev-parse", LAST_COMMIT_CMD], {
-                    encoding: "utf-8",
-                });
-                BASE_SHA = baseRes.stdout;
-                core.setOutput("noPreviousBuild", "true");
+                if (fallbackSHA) {
+                    BASE_SHA = fallbackSHA;
+                    process.stdout.write(`Using provided fallback SHA: ${fallbackSHA}\n`);
+                }
+                else {
+                    process.stdout.write(`We are therefore defaulting to use HEAD~1 on 'origin/${mainBranchName}'\n`);
+                    process.stdout.write("\n");
+                    process.stdout.write(`NOTE: You can instead make this a hard error by setting 'error-on-no-successful-workflow' on the action in your workflow.\n`);
+                    process.stdout.write("\n");
+                    const commitCountOutput = (0, child_process_1.spawnSync)("git", ["rev-list", "--count", `origin/${mainBranchName}`], { encoding: "utf-8" }).stdout;
+                    const commitCount = parseInt(stripNewLineEndings(commitCountOutput), 10);
+                    const LAST_COMMIT_CMD = `origin/${mainBranchName}${commitCount > 1 ? "~1" : ""}`;
+                    const baseRes = (0, child_process_1.spawnSync)("git", ["rev-parse", LAST_COMMIT_CMD], {
+                        encoding: "utf-8",
+                    });
+                    BASE_SHA = baseRes.stdout;
+                    core.setOutput("noPreviousBuild", "true");
+                }
             }
         }
         else {
diff --git a/find-successful-workflow.ts b/find-successful-workflow.ts
index a0e1ee1..fd95d2f 100644
--- a/find-successful-workflow.ts
+++ b/find-successful-workflow.ts
@@ -17,6 +17,7 @@ const errorOnNoSuccessfulWorkflow = process.argv[4];
 const lastSuccessfulEvent = process.argv[5];
 const workingDirectory = process.argv[6];
 const workflowId = process.argv[7];
+const fallbackSHA = process.argv[8];
 const defaultWorkingDirectory = ".";
 
 const ProxifiedClient = Octokit.plugin(proxyPlugin);
@@ -29,7 +30,7 @@ let BASE_SHA: string;
     } else {
       process.stdout.write("\n");
       process.stdout.write(
-        `WARNING: Working directory '${workingDirectory}' doesn't exist.\n`
+        `WARNING: Working directory '${workingDirectory}' doesn't exist.\n`,
       );
     }
   }
@@ -49,7 +50,7 @@ let BASE_SHA: string;
       const baseResult = spawnSync(
         "git",
         ["merge-base", `origin/${mainBranchName}`, mergeBaseRef],
-        { encoding: "utf-8" }
+        { encoding: "utf-8" },
       );
       BASE_SHA = baseResult.stdout;
     } catch (e) {
@@ -64,7 +65,7 @@ let BASE_SHA: string;
         owner,
         repo,
         mainBranchName,
-        lastSuccessfulEvent
+        lastSuccessfulEvent,
       );
     } catch (e) {
       core.setFailed(e.message);
@@ -76,42 +77,47 @@ let BASE_SHA: string;
         reportFailure(mainBranchName);
         return;
       } else {
-        process.stdout.write(   "\n");
-        process.stdout.write(
-          `WARNING: Unable to find a successful workflow run on 'origin/${mainBranchName}', or the latest successful workflow was connected to a commit which no longer exists on that branch (e.g. if that branch was rebased)\n`
-        );
-        process.stdout.write(
-          `We are therefore defaulting to use HEAD~1 on 'origin/${mainBranchName}'\n`
-        );
         process.stdout.write("\n");
         process.stdout.write(
-          `NOTE: You can instead make this a hard error by setting 'error-on-no-successful-workflow' on the action in your workflow.\n`
+          `WARNING: Unable to find a successful workflow run on 'origin/${mainBranchName}', or the latest successful workflow was connected to a commit which no longer exists on that branch (e.g. if that branch was rebased)\n`,
         );
-        process.stdout.write("\n");
+        if (fallbackSHA) {
+          BASE_SHA = fallbackSHA;
+          process.stdout.write(`Using provided fallback SHA: ${fallbackSHA}\n`);
+        } else {
+          process.stdout.write(
+            `We are therefore defaulting to use HEAD~1 on 'origin/${mainBranchName}'\n`,
+          );
+          process.stdout.write("\n");
+          process.stdout.write(
+            `NOTE: You can instead make this a hard error by setting 'error-on-no-successful-workflow' on the action in your workflow.\n`,
+          );
+          process.stdout.write("\n");
 
-        const commitCountOutput = spawnSync(
-          "git",
-          ["rev-list", "--count", `origin/${mainBranchName}`],
-          { encoding: "utf-8" }
-        ).stdout;
-        const commitCount = parseInt(
-          stripNewLineEndings(commitCountOutput),
-          10
-        );
+          const commitCountOutput = spawnSync(
+            "git",
+            ["rev-list", "--count", `origin/${mainBranchName}`],
+            { encoding: "utf-8" },
+          ).stdout;
+          const commitCount = parseInt(
+            stripNewLineEndings(commitCountOutput),
+            10,
+          );
 
-        const LAST_COMMIT_CMD = `origin/${mainBranchName}${
-          commitCount > 1 ? "~1" : ""
-        }`;
-        const baseRes = spawnSync("git", ["rev-parse", LAST_COMMIT_CMD], {
-          encoding: "utf-8",
-        });
-        BASE_SHA = baseRes.stdout;
-        core.setOutput("noPreviousBuild", "true");
+          const LAST_COMMIT_CMD = `origin/${mainBranchName}${
+            commitCount > 1 ? "~1" : ""
+          }`;
+          const baseRes = spawnSync("git", ["rev-parse", LAST_COMMIT_CMD], {
+            encoding: "utf-8",
+          });
+          BASE_SHA = baseRes.stdout;
+          core.setOutput("noPreviousBuild", "true");
+        }
       }
     } else {
       process.stdout.write("\n");
       process.stdout.write(
-        `Found the last successful workflow run on 'origin/${mainBranchName}'\n`
+        `Found the last successful workflow run on 'origin/${mainBranchName}'\n`,
       );
       process.stdout.write(`Commit: ${BASE_SHA}\n`);
     }
@@ -148,7 +154,7 @@ async function findSuccessfulCommit(
   owner: string,
   repo: string,
   branch: string,
-  lastSuccessfulEvent: string
+  lastSuccessfulEvent: string,
 ): Promise<string | undefined> {
   const octokit = new ProxifiedClient();
   if (!workflow_id) {
@@ -162,7 +168,7 @@ async function findSuccessfulCommit(
       .then(({ data: { workflow_id } }) => workflow_id);
     process.stdout.write("\n");
     process.stdout.write(
-      `Workflow Id not provided. Using workflow '${workflow_id}'\n`
+      `Workflow Id not provided. Using workflow '${workflow_id}'\n`,
     );
   }
   // fetch all workflow runs on a given repo/branch/workflow with push and success
@@ -177,10 +183,10 @@ async function findSuccessfulCommit(
         workflow_id,
         event: lastSuccessfulEvent,
         status: "success",
-      }
+      },
     )
     .then(({ data: { workflow_runs } }) =>
-      workflow_runs.map((run: { head_sha: any }) => run.head_sha)
+      workflow_runs.map((run: { head_sha: any }) => run.head_sha),
     );
 
   return await findExistingCommit(octokit, branch, shas);
@@ -198,7 +204,7 @@ async function findMergeBaseRef(): Promise<string> {
 function findMergeQueuePr(): string {
   const { head_ref, base_sha } = github.context.payload.merge_group;
   const result = new RegExp(
-    `^refs/heads/gh-readonly-queue/${mainBranchName}/pr-(\\d+)-${base_sha}$`
+    `^refs/heads/gh-readonly-queue/${mainBranchName}/pr-(\\d+)-${base_sha}$`,
   ).exec(head_ref);
   return result ? result.at(1) : undefined;
 }
@@ -213,7 +219,7 @@ async function findMergeQueueBranch(): Promise<string> {
   const octokit = new ProxifiedClient();
   const result = await octokit.request(
     `GET /repos/${owner}/${repo}/pulls/${pull_number}`,
-    { owner, repo, pull_number: +pull_number }
+    { owner, repo, pull_number: +pull_number },
   );
   return result.data.head.ref;
 }
@@ -224,7 +230,7 @@ async function findMergeQueueBranch(): Promise<string> {
 async function findExistingCommit(
   octokit: Octokit,
   branchName: string,
-  shas: string[]
+  shas: string[],
 ): Promise<string | undefined> {
   for (const commitSha of shas) {
     if (await commitExists(octokit, branchName, commitSha)) {
@@ -240,7 +246,7 @@ async function findExistingCommit(
 async function commitExists(
   octokit: Octokit,
   branchName: string,
-  commitSha: string
+  commitSha: string,
 ): Promise<boolean> {
   try {
     spawnSync("git", ["cat-file", "-e", commitSha], {
@@ -263,7 +269,7 @@ async function commitExists(
     });
 
     return commits.data.some(
-      (commit: { sha: string }) => commit.sha === commitSha
+      (commit: { sha: string }) => commit.sha === commitSha,
     );
   } catch {
     return false;