diff --git a/crates/fj-core/src/validation/checks/half_edge_connection.rs b/crates/fj-core/src/validation/checks/half_edge_connection.rs
new file mode 100644
index 000000000..296f5ac86
--- /dev/null
+++ b/crates/fj-core/src/validation/checks/half_edge_connection.rs
@@ -0,0 +1,101 @@
+use fj_math::{Point, Scalar};
+
+use crate::{
+    objects::{Cycle, HalfEdge},
+    storage::Handle,
+    validation::{validation_check::ValidationCheck, ValidationConfig},
+};
+
+/// Adjacent [`HalfEdge`]s in [`Cycle`] are not connected
+///
+/// Each [`HalfEdge`] only references its start vertex. The end vertex is always
+/// assumed to be the start vertex of the next [`HalfEdge`] in the cycle. This
+/// part of the definition carries no redundancy, and thus doesn't need to be
+/// subject to a validation check.
+///
+/// However, the *position* of that shared vertex is redundantly defined in both
+/// [`HalfEdge`]s. This check verifies that both positions are the same.
+#[derive(Clone, Debug, thiserror::Error)]
+#[error(
+    "Adjacent `HalfEdge`s in `Cycle` are not connected\n\
+    - End position of first `HalfEdge`: {end_pos_of_first_half_edge:?}\n\
+    - Start position of second `HalfEdge`: {start_pos_of_second_half_edge:?}\n\
+    - Distance between vertices: {distance_between_positions}\n\
+    - The unconnected `HalfEdge`s: {unconnected_half_edges:#?}"
+)]
+pub struct AdjacentHalfEdgesNotConnected {
+    /// The end position of the first [`HalfEdge`]
+    pub end_pos_of_first_half_edge: Point<2>,
+
+    /// The start position of the second [`HalfEdge`]
+    pub start_pos_of_second_half_edge: Point<2>,
+
+    /// The distance between the two positions
+    pub distance_between_positions: Scalar,
+
+    /// The edges
+    pub unconnected_half_edges: [Handle<HalfEdge>; 2],
+}
+
+impl ValidationCheck<AdjacentHalfEdgesNotConnected> for Cycle {
+    fn check(
+        &self,
+        config: &ValidationConfig,
+    ) -> impl Iterator<Item = AdjacentHalfEdgesNotConnected> {
+        self.half_edges().pairs().filter_map(|(first, second)| {
+            let end_pos_of_first_half_edge = {
+                let [_, end] = first.boundary().inner;
+                first.path().point_from_path_coords(end)
+            };
+            let start_pos_of_second_half_edge = second.start_position();
+
+            let distance_between_positions = (end_pos_of_first_half_edge
+                - start_pos_of_second_half_edge)
+                .magnitude();
+
+            if distance_between_positions > config.identical_max_distance {
+                return Some(AdjacentHalfEdgesNotConnected {
+                    end_pos_of_first_half_edge,
+                    start_pos_of_second_half_edge,
+                    distance_between_positions,
+                    unconnected_half_edges: [first.clone(), second.clone()],
+                });
+            }
+
+            None
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use crate::{
+        objects::{Cycle, HalfEdge},
+        operations::{
+            build::{BuildCycle, BuildHalfEdge},
+            update::UpdateCycle,
+        },
+        validation::ValidationCheck,
+        Core,
+    };
+
+    #[test]
+    fn adjacent_half_edges_connected() -> anyhow::Result<()> {
+        let mut core = Core::new();
+
+        let valid = Cycle::polygon([[0., 0.], [1., 0.], [1., 1.]], &mut core);
+        valid.check_and_return_first_error()?;
+
+        let invalid = valid.update_half_edge(
+            valid.half_edges().first(),
+            |_, core| {
+                [HalfEdge::line_segment([[0., 0.], [2., 0.]], None, core)]
+            },
+            &mut core,
+        );
+        invalid.check_and_expect_one_error();
+
+        Ok(())
+    }
+}
diff --git a/crates/fj-core/src/validation/checks/mod.rs b/crates/fj-core/src/validation/checks/mod.rs
index 0512298b2..ab9e9debd 100644
--- a/crates/fj-core/src/validation/checks/mod.rs
+++ b/crates/fj-core/src/validation/checks/mod.rs
@@ -1,3 +1,7 @@
 //! All validation checks
 //!
 //! See documentation of [parent module](super) for more information.
+
+mod half_edge_connection;
+
+pub use self::half_edge_connection::AdjacentHalfEdgesNotConnected;