diff --git a/CHANGELOG.md b/CHANGELOG.md
index 55f003b056..a7056afed3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -49,6 +49,7 @@ release.
 - Fixed a bug in which 'version' file was compiled as source and prevented subsequent ISIS recompilation [#5374](https://github.com/DOI-USGS/ISIS3/issues/5374)
 - Fixed <i>noproj</i> bug where some temporary files were not deleted after call to cam2cam.  Issue: [#4813](https://github.com/USGS-Astrogeology/ISIS3/issues/4813)
 - Fixed <i>noproj</i> bug where missing shapemodel-related keywords (RayTraceEngine, BulletParts, Tolerance) are dropped when the output label is created. This resulted in the Bullet collision detection engine not being used. Issue: [#5377](https://github.com/USGS-Astrogeology/ISIS3/issues/5377)
+- Fixed ISIS failing to expand env variables with an "_" in them. [#5402](https://github.com/DOI-USGS/ISIS3/pull/5402)
 
 ## [8.0.2] - 2023-12-05
 ### Changed
diff --git a/isis/src/base/objs/FileName/FileName.cpp b/isis/src/base/objs/FileName/FileName.cpp
index 8927da9636..425e0801a5 100644
--- a/isis/src/base/objs/FileName/FileName.cpp
+++ b/isis/src/base/objs/FileName/FileName.cpp
@@ -846,7 +846,7 @@ namespace Isis {
     // Loop while there are any "$" at the current position or after
     // Some "$" might be skipped if no translation can be found
     while((varStartPos = expandedStr.indexOf("$", varSearchStartPos)) != -1) {
-      int varEndPos = expandedStr.indexOf(QRegExp("[^a-zA-Z{}0-9]"), varStartPos + 1);
+      int varEndPos = expandedStr.indexOf(QRegExp("[^a-zA-Z{}0-9_]"), varStartPos + 1);
       if (varEndPos == -1)
         varEndPos = expandedStr.length();
 
diff --git a/isis/tests/FileNameTests.cpp b/isis/tests/FileNameTests.cpp
index fc8aa4b345..ce19019a71 100644
--- a/isis/tests/FileNameTests.cpp
+++ b/isis/tests/FileNameTests.cpp
@@ -84,13 +84,21 @@ TEST(FileName, Extension) {
   EXPECT_EQ("cub", file.extension());
 }
 
-TEST(FileName, Expanded) {
+TEST(FileName, ExpandedDefault) {
   QString relativeFileName("test.cub");
   FileName file("$ISISROOT/" + relativeFileName);
   QString isisRoot(getenv("ISISROOT"));
   EXPECT_EQ(isisRoot + "/" + relativeFileName, file.expanded());
 }
 
+TEST(FileName, ExpandedUnderscore) {
+  QString relativeFileName("test.cub");
+  setenv("SOME_FILE_PATH", getenv("ISISROOT"), 1);
+  FileName file("$SOME_FILE_PATH/" + relativeFileName);
+  QString someFilePath(getenv("ISISROOT"));
+  EXPECT_EQ(someFilePath + "/" + relativeFileName, file.expanded());
+}
+
 TEST(FileName, Original) {
   QString test = "$ISISROOT/testy/mc/test/face/test.cub";
   FileName file(test);