diff --git a/src/resolution.rs b/src/resolution.rs
index 9cc665f..f7507ec 100644
--- a/src/resolution.rs
+++ b/src/resolution.rs
@@ -1,9 +1,17 @@
 use crate::{DependencyGroupSpecifier, DependencyGroups, ResolvedDependencies};
 use indexmap::IndexMap;
-use pep508_rs::Requirement;
+use pep508_rs::{ExtraName, Requirement};
 use std::fmt::Display;
+use std::str::FromStr;
 use thiserror::Error;
 
+/// Normalize a group/extra name according to PEP 685.
+fn normalize_name(name: &str) -> String {
+    ExtraName::from_str(name)
+        .map(|extra| extra.to_string())
+        .unwrap_or_else(|_| name.to_string())
+}
+
 #[derive(Debug, Error)]
 #[error(transparent)]
 pub struct ResolveError(#[from] ResolveErrorKind);
@@ -105,7 +113,16 @@ fn resolve_optional_dependency(
         return Ok(requirements.clone());
     }
 
-    let Some(unresolved_requirements) = optional_dependencies.get(extra) else {
+    let normalized_extra = normalize_name(extra);
+
+    // Find the key in optional_dependencies by comparing normalized versions
+    // TODO: next breaking release remove this once Extra is added
+    let unresolved_requirements = optional_dependencies
+        .iter()
+        .find(|(key, _)| normalize_name(key) == normalized_extra)
+        .map(|(_, reqs)| reqs);
+
+    let Some(unresolved_requirements) = unresolved_requirements else {
         let parent = parents
             .iter()
             .last()
@@ -460,4 +477,38 @@ mod tests {
             vec![Requirement::from_str("numpy").unwrap()]
         );
     }
+
+    #[test]
+    fn optional_dependencies_with_underscores() {
+        // Test that optional dependency group names with underscores are normalized
+        // when referenced in extras. PEP 685 specifies that extras should be normalized
+        // by replacing _, ., - with a single -.
+        let source = r#"
+            [project]
+            name = "foo"
+
+            [project.optional-dependencies]
+            all = [
+              "foo[group-one]",
+              "foo[group_two]",
+            ]
+            group_one = [
+              "anyio>=4.9.0",
+            ]
+            group-two = [
+              "trio>=0.31.0",
+            ]
+        "#;
+        let pyproject_toml = PyProjectToml::new(source).unwrap();
+        let resolved_dependencies = pyproject_toml.resolve().unwrap();
+
+        // Both group-one and group_two should resolve correctly
+        assert_eq!(
+            resolved_dependencies.optional_dependencies["all"],
+            vec![
+                Requirement::from_str("anyio>=4.9.0").unwrap(),
+                Requirement::from_str("trio>=0.31.0").unwrap(),
+            ]
+        );
+    }
 }