diff --git a/Frontmatter.Test/LineEndingTests.cs b/Frontmatter.Test/LineEndingTests.cs
new file mode 100644
index 0000000..8f6f77c
--- /dev/null
+++ b/Frontmatter.Test/LineEndingTests.cs
@@ -0,0 +1,86 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Tests that every writer emits the input document's line ending, whatever the host's is.
+/// </summary>
+[TestClass]
+public class LineEndingTests
+{
+	private static string Lines(string newLine, params string[] lines) => string.Join(newLine, lines) + newLine;
+
+	private static void AssertOnlyLineEnding(string newLine, string text)
+	{
+		string withoutLineEndings = text.Replace(newLine, string.Empty, StringComparison.Ordinal);
+		Assert.DoesNotContain("\r", withoutLineEndings);
+		Assert.DoesNotContain("\n", withoutLineEndings);
+	}
+
+	[TestMethod]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\n", DisplayName = "LF")]
+	public void CombineFrontmatter_KeepsTheInputLineEnding(string nl)
+	{
+		string input = Lines(nl, "---", "title: A", "---", "---", "author: B", "---", "line1", "line2");
+
+		string result = Frontmatter.CombineFrontmatter(input, FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, FrontmatterMergeStrategy.None);
+
+		Assert.AreEqual(Lines(nl, "---", "title: A", "author: B", "---", "line1", "line2"), result);
+		AssertOnlyLineEnding(nl, result);
+	}
+
+	[TestMethod]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\n", DisplayName = "LF")]
+	public void AddFrontmatter_DocumentWithoutHeader_KeepsTheInputLineEnding(string nl)
+	{
+		string input = Lines(nl, "line1", "line2");
+
+		string result = Frontmatter.AddFrontmatter(input, new Dictionary<string, object> { ["title"] = "T", ["author"] = "B" });
+
+		Assert.AreEqual(Lines(nl, "---", "title: T", "author: B", "---", "line1", "line2"), result);
+		AssertOnlyLineEnding(nl, result);
+	}
+
+	[TestMethod]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\n", DisplayName = "LF")]
+	public void AddFrontmatter_DocumentWithHeader_KeepsTheInputLineEnding(string nl)
+	{
+		string input = Lines(nl, "---", "title: A", "---", "line1", "line2");
+
+		string result = Frontmatter.AddFrontmatter(input, new Dictionary<string, object> { ["author"] = "B" });
+
+		Assert.AreEqual(Lines(nl, "---", "title: A", "author: B", "---", "line1", "line2"), result);
+		AssertOnlyLineEnding(nl, result);
+	}
+
+	[TestMethod]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\n", DisplayName = "LF")]
+	public void ReplaceFrontmatter_KeepsTheInputLineEnding(string nl)
+	{
+		string input = Lines(nl, "---", "title: A", "---", "line1", "line2");
+
+		string result = Frontmatter.ReplaceFrontmatter(input, new Dictionary<string, object> { ["title"] = "B", ["tags"] = new List<object> { "x", "y" } });
+
+		Assert.StartsWith($"---{nl}title: B{nl}tags:{nl}", result);
+		Assert.EndsWith($"{nl}---{nl}line1{nl}line2{nl}", result);
+		AssertOnlyLineEnding(nl, result);
+	}
+
+	[TestMethod]
+	[DataRow("\r\n", DisplayName = "CRLF")]
+	[DataRow("\n", DisplayName = "LF")]
+	public void RemoveFrontmatter_KeepsTheInputLineEnding(string nl)
+	{
+		string input = Lines(nl, "---", "title: A", "---", "line1", "line2");
+
+		string result = Frontmatter.RemoveFrontmatter(input);
+
+		Assert.AreEqual(Lines(nl, "line1", "line2"), result);
+	}
+}
diff --git a/Frontmatter/Frontmatter.cs b/Frontmatter/Frontmatter.cs
index 76b1f83..04dcb52 100644
--- a/Frontmatter/Frontmatter.cs
+++ b/Frontmatter/Frontmatter.cs
@@ -5,6 +5,7 @@ namespace ktsu.Frontmatter;
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Text;
 
 /// <summary>
 /// Provides methods for processing and manipulating YAML frontmatter in markdown files.
@@ -110,9 +111,9 @@ public static string CombineFrontmatter(string input, FrontmatterNaming property
 			combinedFrontmatterObject = SortFrontmatterProperties(combinedFrontmatterObject);
 		}
 
-		string combinedFrontmatter = YamlSerializer.SerializeYamlObject(combinedFrontmatterObject).Trim();
+		string nl = NewLineOf(input);
+		string combinedFrontmatter = WithNewLines(YamlSerializer.SerializeYamlObject(combinedFrontmatterObject).Trim(), nl);
 
-		string nl = Environment.NewLine;
 		string result = $"{FrontmatterDelimiter}{nl}{combinedFrontmatter}{nl}{FrontmatterDelimiter}{nl}{body}";
 
 		// Cache the processed result
@@ -197,8 +198,8 @@ public static string AddFrontmatter(string input, Dictionary<string, object> fro
 			return ReplaceFrontmatter(input, combined);
 		}
 
-		string yamlFrontmatter = YamlSerializer.SerializeYamlObject(frontmatter).Trim();
-		string nl = Environment.NewLine;
+		string nl = NewLineOf(input);
+		string yamlFrontmatter = WithNewLines(YamlSerializer.SerializeYamlObject(frontmatter).Trim(), nl);
 		return $"{FrontmatterDelimiter}{nl}{yamlFrontmatter}{nl}{FrontmatterDelimiter}{nl}{TrimBody(input)}{nl}";
 	}
 
@@ -219,8 +220,8 @@ public static string ReplaceFrontmatter(string input, Dictionary<string, object>
 		}
 
 		ExtractFrontmatterObjects(input, out string body);
-		string yamlFrontmatter = YamlSerializer.SerializeYamlObject(frontmatter).Trim();
-		string nl = Environment.NewLine;
+		string nl = NewLineOf(input);
+		string yamlFrontmatter = WithNewLines(YamlSerializer.SerializeYamlObject(frontmatter).Trim(), nl);
 		return $"{FrontmatterDelimiter}{nl}{yamlFrontmatter}{nl}{FrontmatterDelimiter}{nl}{TrimBody(body)}{nl}";
 	}
 
@@ -240,7 +241,7 @@ public static string RemoveFrontmatter(string input)
 		}
 
 		ExtractFrontmatterObjects(input, out string body);
-		return TrimBody(body) + Environment.NewLine;
+		return TrimBody(body) + NewLineOf(input);
 	}
 
 	/// <summary>
@@ -534,6 +535,53 @@ private static string TrimBody(string body)
 	private static bool IsClosingDelimiterLine(string line) =>
 		IsDelimiterLine(line) || line.TrimEnd() == DocumentEndMarker;
 
+	/// <summary>
+	/// Finds the line ending a document uses, so a rewritten document keeps it rather than mixing in the
+	/// host's.
+	/// </summary>
+	/// <param name="input">The document to inspect.</param>
+	/// <returns>
+	/// The document's first line ending, CRLF, LF or CR, or <see cref="Environment.NewLine"/> when it has none.
+	/// </returns>
+	private static string NewLineOf(string input)
+	{
+		int index = input.IndexOfAny(['\r', '\n']);
+		if (index < 0)
+		{
+			return Environment.NewLine;
+		}
+
+		if (input[index] == '\n')
+		{
+			return "\n";
+		}
+
+		return index + 1 < input.Length && input[index + 1] == '\n' ? "\r\n" : "\r";
+	}
+
+	/// <summary>
+	/// Rewrites every line ending in a piece of text to the given one.
+	/// </summary>
+	/// <param name="text">The text to rewrite, such as serialized YAML.</param>
+	/// <param name="newLine">The line ending to use.</param>
+	/// <returns>The text with only <paramref name="newLine"/> line endings.</returns>
+	private static string WithNewLines(string text, string newLine)
+	{
+		List<(int Start, int End)> lines = SplitLines(text);
+		StringBuilder builder = new(text.Length);
+		for (int i = 0; i < lines.Count; i++)
+		{
+			if (i > 0)
+			{
+				builder.Append(newLine);
+			}
+
+			builder.Append(text, lines[i].Start, lines[i].End - lines[i].Start);
+		}
+
+		return builder.ToString();
+	}
+
 	/// <summary>
 	/// Combines two frontmatter dictionaries into a single dictionary.
 	/// </summary>