diff --git a/opensfm/config.py b/opensfm/config.py
index 372efcc04..bdf30c0da 100644
--- a/opensfm/config.py
+++ b/opensfm/config.py
@@ -111,6 +111,9 @@ class OpenSfMConfig:
     flann_tree: int = 8
     # Smaller -> Faster (but might lose good matches)
     flann_checks: int = 20
+    # Seed for the RNG that FLANN's index construction draws from.
+    # Fixed by default so matching is reproducible; see build_flann_index.
+    flann_random_seed: int = 42
 
     ##################################
     # Params for BoW matching
diff --git a/opensfm/features.py b/opensfm/features.py
index 0fb752231..f87454522 100644
--- a/opensfm/features.py
+++ b/opensfm/features.py
@@ -670,4 +670,11 @@ def build_flann_index(descriptors: NDArray, config: Dict[str, Any]) -> cv2.flann
         )
 
     # pyrefly: ignore [not-callable]
+    # Pin OpenCV's RNG so the index is a pure function of (descriptors,
+    # seed). FLANN's index construction draws from cv::theRNG(), which is
+    # thread-local and advances between builds, so without this two workers
+    # build different indexes from identical descriptors and matching is
+    # not reproducible.
+    cv2.setRNGSeed(config["flann_random_seed"])
+
     return context.flann_Index(descriptors, flann_params)