--- a/src/gromacs/fileio/groio.cpp
+++ b/src/gromacs/fileio/groio.cpp
@@ -341,7 +341,15 @@
 
     if (!beof)
     {
-        gmx_fseek(fp, -1, SEEK_CUR);
+        /* Push the peeked byte back with ungetc instead of seeking backwards.
+         * The original gmx_fseek(fp, -1, SEEK_CUR) fails with EINVAL on
+         * text-mode streams with the Windows CRT (relative seeks are not
+         * supported there), which broke reading of all multi-frame .gro
+         * trajectories on Windows. ungetc is standard and mode-independent. */
+        if (ungetc(data[0], fp) == EOF)
+        {
+            gmx_fseek(fp, -1, SEEK_CUR);
+        }
     }
     return beof;
 }