diff --git a/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md b/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md
index eb19f4014c8..b89aadb1915 100644
--- a/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md
+++ b/src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md
@@ -7,6 +7,10 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Fixed metric unit strings containing invalid Prometheus characters (e.g. `# RU`)
+  not being sanitized, resulting in malformed metric names.
+  ([#6187](https://github.com/open-telemetry/opentelemetry-dotnet/issues/6187))
+
 ## 1.15.2-beta.1
 
 Released 2026-Apr-08
diff --git a/src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md b/src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md
index 477525dbe4d..1c19651dc39 100644
--- a/src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md
+++ b/src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md
@@ -7,6 +7,10 @@ Notes](../../RELEASENOTES.md).
 
 ## Unreleased
 
+* Fixed metric unit strings containing invalid Prometheus characters (e.g. `# RU`)
+  not being sanitized, resulting in malformed metric names.
+  ([#6187](https://github.com/open-telemetry/opentelemetry-dotnet/issues/6187))
+
 ## 1.15.2-beta.1
 
 Released 2026-Apr-08
diff --git a/src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs b/src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs
index df1632ca467..73665c7a75a 100644
--- a/src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs
+++ b/src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusMetric.cs
@@ -77,6 +77,39 @@ public PrometheusMetric(string name, string unit, PrometheusType type, bool disa
     public static PrometheusMetric Create(Metric metric, bool disableTotalNameSuffixForCounters)
         => new(metric.Name, metric.Unit, GetPrometheusType(metric.MetricType), disableTotalNameSuffixForCounters);
 
+    internal static string SanitizeMetricUnit(string metricUnit)
+    {
+        StringBuilder? sb = null;
+        var lastCharUnderscore = false;
+
+        for (var i = 0; i < metricUnit.Length; i++)
+        {
+            var c = metricUnit[i];
+
+            if (!char.IsLetterOrDigit(c) && c != ':')
+            {
+                if (!lastCharUnderscore)
+                {
+                    lastCharUnderscore = true;
+                    sb ??= new StringBuilder(metricUnit, 0, i, metricUnit.Length);
+                    sb.Append('_');
+                }
+            }
+            else
+            {
+                if (sb != null)
+                {
+                    sb.Append(c);
+                }
+
+                lastCharUnderscore = false;
+            }
+        }
+
+        var result = sb?.ToString() ?? metricUnit;
+        return result.Trim('_');
+    }
+
     internal static string SanitizeMetricName(string metricName)
     {
         StringBuilder? sb = null;
@@ -112,11 +145,6 @@ internal static string SanitizeMetricName(string metricName)
         }
 
         return sb?.ToString() ?? metricName;
-
-        static StringBuilder CreateStringBuilder(string name)
-        {
-            return new(name.Length);
-        }
     }
 
     internal static string RemoveAnnotations(string unit)
@@ -186,6 +214,11 @@ UpDownCounter becomes gauge
         };
     }
 
+    private static StringBuilder CreateStringBuilder(string value)
+    {
+        return new(value.Length);
+    }
+
     private static string SanitizeOpenMetricsName(string metricName)
         => metricName.EndsWith("_total", StringComparison.Ordinal) ? metricName.Substring(0, metricName.Length - 6) : metricName;
 
@@ -208,7 +241,7 @@ private static string GetUnit(string unit)
             updatedUnit = MapUnit(updatedUnit.AsSpan());
         }
 
-        return updatedUnit;
+        return SanitizeMetricUnit(updatedUnit);
     }
 
     private static bool TryProcessRateUnits(string updatedUnit, [NotNullWhen(true)] out string? updatedPerUnit)
diff --git a/test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs b/test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs
index 5e8856f4843..785253c25a2 100644
--- a/test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs
+++ b/test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusMetricTests.cs
@@ -361,7 +361,43 @@ public void GetPrometheusType_HistogramVariants_ReturnsHistogram(int metricTypeV
 
     [Fact]
     public void Name_MultipleSlashesInUnit_FirstSlashProcessed()
-        => AssertName("metric", "req/s/extra", PrometheusType.Gauge, false, "metric_req_per_s/extra"); // // Multiple slashes
+        => AssertName("metric", "req/s/extra", PrometheusType.Gauge, false, "metric_req_per_s_extra"); // // Multiple slashes
+
+    [Fact]
+    public void SanitizeMetricUnit_Valid()
+        => AssertSanitizeMetricUnit("requests", "requests");
+
+    [Fact]
+    public void SanitizeMetricUnit_RemoveConsecutiveUnderscores()
+        => AssertSanitizeMetricUnit("req__per__s", "req_per_s");
+
+    [Fact]
+    public void SanitizeMetricUnit_RemoveUnsupportedCharacters()
+        => AssertSanitizeMetricUnit("# RU", "RU");
+
+    [Fact]
+    public void SanitizeMetricUnit_RemoveWhitespace()
+        => AssertSanitizeMetricUnit("req s", "req_s");
+
+    [Fact]
+    public void SanitizeMetricUnit_LeadingNumberAllowed()
+        => AssertSanitizeMetricUnit("2_unitname", "2_unitname");
+
+    [Fact]
+    public void SanitizeMetricUnit_RemoveMultipleUnsupportedCharacters()
+        => AssertSanitizeMetricUnit("##/RU!", "RU");
+
+    [Fact]
+    public void Name_UnitWithHash_Sanitized()
+        => AssertName("azure_cosmosdb_client_operation_request_charge", "# RU", PrometheusType.Histogram, false, "azure_cosmosdb_client_operation_request_charge_RU");
+
+    [Fact]
+    public void Name_UnitWithSpace_Sanitized()
+        => AssertName("metric", "req s", PrometheusType.Gauge, false, "metric_req_s");
+
+    [Fact]
+    public void Name_UnitWithSpecialChars_Sanitized()
+        => AssertName("metric", "req!", PrometheusType.Gauge, false, "metric_req");
 
     [Theory]
     [InlineData(PrometheusType.Counter)]
@@ -375,6 +411,12 @@ internal void Constructor_AllPrometheusTypes_Work(PrometheusType type)
         Assert.Equal(type, metric.Type);
     }
 
+    private static void AssertSanitizeMetricUnit(string unit, string expected)
+    {
+        var sanitizedUnit = PrometheusMetric.SanitizeMetricUnit(unit);
+        Assert.Equal(expected, sanitizedUnit);
+    }
+
     private static void AssertName(
         string name, string unit, PrometheusType type, bool disableTotalNameSuffixForCounters, string expected)
     {