diff --git a/src/changelog/3.5.1/338-size-backup-ignores-prefix-sibling.xml b/src/changelog/3.5.1/338-size-backup-ignores-prefix-sibling.xml
new file mode 100644
index 00000000..3b7cbcb8
--- /dev/null
+++ b/src/changelog/3.5.1/338-size-backup-ignores-prefix-sibling.xml
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="338" link="https://github.com/apache/logging-log4net/issues/338"/>
+  <issue id="339" link="https://github.com/apache/logging-log4net/pull/339"/>
+  <description format="asciidoc">Without date rolling, a `RollingFileAppender` with `StaticLogFileName`
+  false counted a file such as `log.txt.errors.3` as backup 3 of `log.txt` when it started. Only
+  files whose name is the base name followed by the index count as backups now (reported by
+  @dajiaohuang, fixed by @dajiaohuang)</description>
+</entry>
diff --git a/src/log4net.Tests/Appender/RollingFileAppenderTest.cs b/src/log4net.Tests/Appender/RollingFileAppenderTest.cs
index 8ef01257..588436c9 100644
--- a/src/log4net.Tests/Appender/RollingFileAppenderTest.cs
+++ b/src/log4net.Tests/Appender/RollingFileAppenderTest.cs
@@ -1799,6 +1799,30 @@ public void TestInitializeRollBackupsNotStaticDateWithoutDashes(string baseFile,
     Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(1));
   }
 
+  /// <summary>
+  /// Tests that without date rolling a sibling such as log.txt.errors.3 is not counted as a backup
+  /// of a non-static name
+  /// </summary>
+  [TestCase(true, "log.5.txt", 5)]
+  [TestCase(false, "log.txt.5", 5)]
+  [TestCase(true, "log.errors.3.txt", 0)]
+  [TestCase(false, "log.txt.errors.3", 0)]
+  public void TestInitializeRollBackupsSizeIgnoresPrefixSibling(bool preserveExtension,
+    string file, int expected)
+  {
+    RollingFileAppender rfa = new()
+    {
+      RollingStyle = RollingFileAppender.RollingMode.Size,
+      StaticLogFileName = false,
+      PreserveLogFileNameExtension = preserveExtension,
+      MaxSizeRollBackups = -1,
+      CurrentSizeRollBackups = 0
+    };
+    rfa.InitializeRollBackups("log.txt", [file]);
+
+    Assert.That(rfa.CurrentSizeRollBackups, Is.EqualTo(expected));
+  }
+
   /// <summary>
   /// Tests that with a short date pattern the suffixes of a static file name count as backup
   /// indexes, as a date like .10 cannot be told apart from backup 10
diff --git a/src/log4net/Appender/RollingFileAppender.cs b/src/log4net/Appender/RollingFileAppender.cs
index 15dc84a7..c6b17a31 100644
--- a/src/log4net/Appender/RollingFileAppender.cs
+++ b/src/log4net/Appender/RollingFileAppender.cs
@@ -971,8 +971,8 @@ private int GetBackupIndex(string baseFile, string curFileName)
     int index = fileName.LastIndexOf(".", StringComparison.Ordinal);
     if (index > 0)
     {
-      // with a static name, log.txt.20261003.1 is a backup of an earlier date
-      if (StaticLogFileName
+      // without dates log.txt.errors.3 is another file (#338)
+      if ((StaticLogFileName || !_rollDate)
         && (!string.Equals(fileName.Substring(0, index), baseFile, StringComparison.Ordinal)
           || IsDateSuffix(fileName.Substring(index))))
       {