diff --git a/server/data/climate_constants.toml b/server/data/climate_constants.toml
index 808c0bee9..864eed01d 100644
--- a/server/data/climate_constants.toml
+++ b/server/data/climate_constants.toml
@@ -65,3 +65,22 @@ standard   = 15   # ~15°C: Earth-like moderate swing
 toxic      = 20   # intermediate: thick but maybe less redistribution
 breathable = 15   # synonym for standard
 dense      = 3    # ~3°C: Venus-like near-uniform temperature
+
+# ---------------------------------------------------------------------------
+[hydrosphere_maritime]
+# Maritime moderation (D-240): scales the equator→pole temperature gradient by
+# hydrosphere. 1.0 = full gradient (dry world swings the whole class band);
+# < 1.0 compresses toward the band midpoint (water-rich worlds are milder at
+# both poles and equator). Keys = actual bodies.hydrosphere vocabulary in
+# systems.db. "minimal"/"trace"/"none"/absent default to 1.0 in code.
+liquid_water      = 0.6   # large surface liquid — strongly moderated
+ocean             = 0.6
+"ocean-coastal"   = 0.6
+extensive         = 0.6
+rivers            = 0.8   # partial surface water — mild moderation
+"rivers-lakes"    = 0.8
+moderate          = 0.8
+ice               = 0.85  # frozen surface — slight moderation
+subsurface_liquid = 0.9
+subsurface        = 0.95  # buried water — minimal surface effect
+subsurface_ice    = 0.95
diff --git a/server/src/atlas/region_profile.rs b/server/src/atlas/region_profile.rs
index b21e3d0f0..a2befac71 100644
--- a/server/src/atlas/region_profile.rs
+++ b/server/src/atlas/region_profile.rs
@@ -634,6 +634,11 @@ pub struct ClimateConstants {
     pub greenhouse_offset_c: std::collections::BTreeMap<String, f32>,
     /// Day/night swing amplitude per atmosphere class (°C).
     pub diurnal_amplitude_c: std::collections::BTreeMap<String, f32>,
+    /// Maritime-moderation factor per `hydrosphere` (D-240): scales the
+    /// equator→pole gradient. `1.0` = full gradient (dry world swings the whole
+    /// band); `< 1.0` compresses toward the band midpoint (water-rich worlds are
+    /// milder at both ends). Absent/unknown = `1.0`.
+    pub hydrosphere_maritime: std::collections::BTreeMap<String, f32>,
 }
 
 impl Default for ClimateConstants {
@@ -669,10 +674,30 @@ impl Default for ClimateConstants {
         da.insert("toxic".into(), 20.0f32);
         da.insert("dense".into(), 3.0f32);
 
+        // Maritime moderation: water-rich worlds compress the equator–pole gradient.
+        // Keys are the actual `bodies.hydrosphere` vocabulary in systems.db.
+        let mut hm = std::collections::BTreeMap::new();
+        // Large surface liquid — strong moderation.
+        hm.insert("liquid_water".into(), 0.6f32);
+        hm.insert("ocean".into(), 0.6f32);
+        hm.insert("ocean-coastal".into(), 0.6f32);
+        hm.insert("extensive".into(), 0.6f32);
+        // Partial surface water — mild moderation.
+        hm.insert("rivers".into(), 0.8f32);
+        hm.insert("rivers-lakes".into(), 0.8f32);
+        hm.insert("moderate".into(), 0.8f32);
+        // Frozen / subsurface — slight moderation.
+        hm.insert("ice".into(), 0.85f32);
+        hm.insert("subsurface_liquid".into(), 0.9f32);
+        hm.insert("subsurface".into(), 0.95f32);
+        hm.insert("subsurface_ice".into(), 0.95f32);
+        // "minimal" / "trace" / "none" / NULL → 1.0 (full gradient) via fallback.
+
         ClimateConstants {
             planet_class_temperature: pct,
             greenhouse_offset_c: gh,
             diurnal_amplitude_c: da,
+            hydrosphere_maritime: hm,
         }
     }
 }
@@ -713,6 +738,16 @@ impl ClimateConstants {
             .unwrap_or(0.0)
     }
 
+    /// Maritime-moderation factor for a given `hydrosphere` (D-240): `1.0` = full
+    /// equator→pole gradient (dry/unknown); `< 1.0` compresses toward the band
+    /// midpoint (ocean ≈ 0.6 → milder at both ends).
+    pub fn maritime_factor(&self, hydrosphere: &str) -> f32 {
+        self.hydrosphere_maritime
+            .get(hydrosphere)
+            .copied()
+            .unwrap_or(1.0)
+    }
+
     /// Diurnal amplitude for a given atmosphere class (°C), defaulting to 30.
     pub fn diurnal_amplitude(&self, atmosphere: &str) -> f32 {
         self.diurnal_amplitude_c
@@ -763,9 +798,16 @@ pub fn derive_temperature_c(
     let (cold, warm) = constants.envelope(planet_class);
     let band_width = warm - cold;
 
-    // Step 2: latitude lerp — equator → warm end, pole → cold end.
+    // Step 2: latitude lerp across the maritime-moderated band (D-240). Water-rich
+    // worlds compress the equator→pole gradient toward the band midpoint — an ocean
+    // world is milder at both ends; a dry world swings the full band.
+    let hydrosphere = params.hydrosphere.as_deref().unwrap_or("none");
+    let maritime = constants.maritime_factor(hydrosphere);
+    let mid = (cold + warm) * 0.5;
+    let half = band_width * 0.5 * maritime;
     let lat_frac = (params.region_latitude_deg.abs() as f32 / 90.0).clamp(0.0, 1.0);
-    let t_lat = warm - band_width * lat_frac;
+    // equator (frac 0) → mid + half; pole (frac 1) → mid − half.
+    let t_lat = (mid + half) - (2.0 * half) * lat_frac;
 
     // Step 3: atmosphere greenhouse nudge — fraction of band_width toward warm end.
     let gh_frac = constants.greenhouse(atmosphere);
@@ -1339,6 +1381,33 @@ mod tests {
         }
     }
 
+    #[test]
+    fn maritime_hydrosphere_compresses_the_gradient() {
+        // D-240: a water-rich world has a SMALLER equator→pole temperature delta
+        // than a dry world of the same class (maritime moderation).
+        let climate = ClimateConstants::default();
+        let mk = |hydro: &str, lat: f64| BodyParams {
+            planet_class: Some("temperate".into()),
+            atmosphere: Some("standard".into()),
+            hydrosphere: Some(hydro.into()),
+            region_latitude_deg: lat,
+            elevation_km: 0.0,
+            ..Default::default()
+        };
+        // Same seed → only hydrosphere differs.
+        let delta = |hydro: &str| {
+            (derive_temperature_c(&mk(hydro, 0.0), &climate, 7).unwrap()
+                - derive_temperature_c(&mk(hydro, 90.0), &climate, 7).unwrap())
+            .abs()
+        };
+        let ocean_delta = delta("ocean");
+        let dry_delta = delta("none"); // hydrosphere none ≠ airless (atmosphere is "standard")
+        assert!(
+            ocean_delta < dry_delta,
+            "ocean gradient {ocean_delta}°C must be milder than dry {dry_delta}°C"
+        );
+    }
+
     #[test]
     fn unknown_planet_class_falls_back_to_temperate_band() {
         // An unrecognised class string must not panic; it falls back to temperate.