diff --git a/src/query/src/promql/planner.rs b/src/query/src/promql/planner.rs
index 5867301f972..972f04b9ccf 100644
--- a/src/query/src/promql/planner.rs
+++ b/src/query/src/promql/planner.rs
@@ -587,9 +587,10 @@ impl PromPlanner {
         };
 
         let current_interval = self.ctx.interval;
-        if let Some(step) = step {
-            self.ctx.interval = step.as_millis() as _;
-        }
+        self.ctx.interval = match step {
+            Some(step) => step.as_millis() as _,
+            None => DEFAULT_SUBQUERY_STEP_MS,
+        };
         ensure!(self.ctx.interval > 0, ZeroRangeSelectorSnafu);
         let current_start = self.ctx.start;
         let current_end = self.ctx.end;
diff --git a/src/query/src/promql/planner/test.rs b/src/query/src/promql/planner/test.rs
index 19de1098288..f78162d8fdb 100644
--- a/src/query/src/promql/planner/test.rs
+++ b/src/query/src/promql/planner/test.rs
@@ -5682,14 +5682,6 @@ async fn subquery_rejects_invalid_range_and_zero_step() {
         matches!(&err, crate::promql::error::Error::ZeroRangeSelector { .. }),
         "{err}"
     );
-    // An omitted child step inherits the zero parent step.
-    let err = plan_subquery_offset_probe("sum_over_time(subquery_offset_offgrid[5s:])", 60, 60, 0)
-        .await
-        .unwrap_err();
-    assert!(
-        matches!(&err, crate::promql::error::Error::ZeroRangeSelector { .. }),
-        "{err}"
-    );
 }
 
 /// Rejects nonempty child windows whose derived millisecond bounds are unrepresentable.
diff --git a/tests/cases/standalone/common/promql/subquery_default_step.result b/tests/cases/standalone/common/promql/subquery_default_step.result
new file mode 100644
index 00000000000..47008660cad
--- /dev/null
+++ b/tests/cases/standalone/common/promql/subquery_default_step.result
@@ -0,0 +1,53 @@
+-- A subquery without a step uses the evaluation interval (one minute by
+-- default in Prometheus), not the query step. Expected results are from Prometheus 3.
+create table subquery_default_step (
+    ts timestamp time index,
+    val double,
+    host string primary key
+);
+
+Affected Rows: 0
+
+insert into subquery_default_step values
+    (0, 1.0, 'a'),
+    (60000, 1.0, 'a'),
+    (120000, 1.0, 'a'),
+    (180000, 1.0, 'a'),
+    (240000, 1.0, 'a'),
+    (300000, 1.0, 'a');
+
+Affected Rows: 6
+
+tql eval (300, 300, '1s') count_over_time(subquery_default_step[5m:]);
+
++---------------------+------------------------------------+------+
+| ts                  | prom_count_over_time(ts_range,val) | host |
++---------------------+------------------------------------+------+
+| 1970-01-01T00:05:00 | 5.0                                | a    |
++---------------------+------------------------------------+------+
+
+tql eval (300, 300, '1s') count_over_time(subquery_default_step[5m:30s]);
+
++---------------------+------------------------------------+------+
+| ts                  | prom_count_over_time(ts_range,val) | host |
++---------------------+------------------------------------+------+
+| 1970-01-01T00:05:00 | 10.0                               | a    |
++---------------------+------------------------------------+------+
+
+-- The outer range query keeps its own step around the subquery's default step.
+tql eval (300, 360, '15s') count_over_time(subquery_default_step[5m:]);
+
++---------------------+------------------------------------+------+
+| ts                  | prom_count_over_time(ts_range,val) | host |
++---------------------+------------------------------------+------+
+| 1970-01-01T00:05:00 | 5.0                                | a    |
+| 1970-01-01T00:05:15 | 5.0                                | a    |
+| 1970-01-01T00:05:30 | 5.0                                | a    |
+| 1970-01-01T00:05:45 | 5.0                                | a    |
+| 1970-01-01T00:06:00 | 5.0                                | a    |
++---------------------+------------------------------------+------+
+
+drop table subquery_default_step;
+
+Affected Rows: 0
+
diff --git a/tests/cases/standalone/common/promql/subquery_default_step.sql b/tests/cases/standalone/common/promql/subquery_default_step.sql
new file mode 100644
index 00000000000..fb1acb5f45b
--- /dev/null
+++ b/tests/cases/standalone/common/promql/subquery_default_step.sql
@@ -0,0 +1,25 @@
+-- A subquery without a step uses the evaluation interval (one minute by
+-- default in Prometheus), not the query step. Expected results are from Prometheus 3.
+
+create table subquery_default_step (
+    ts timestamp time index,
+    val double,
+    host string primary key
+);
+
+insert into subquery_default_step values
+    (0, 1.0, 'a'),
+    (60000, 1.0, 'a'),
+    (120000, 1.0, 'a'),
+    (180000, 1.0, 'a'),
+    (240000, 1.0, 'a'),
+    (300000, 1.0, 'a');
+
+tql eval (300, 300, '1s') count_over_time(subquery_default_step[5m:]);
+
+tql eval (300, 300, '1s') count_over_time(subquery_default_step[5m:30s]);
+
+-- The outer range query keeps its own step around the subquery's default step.
+tql eval (300, 360, '15s') count_over_time(subquery_default_step[5m:]);
+
+drop table subquery_default_step;