diff --git a/packages/lib/data/ModPack.js b/packages/lib/data/ModPack.js
index 286dcea7..8279cf03 100644
--- a/packages/lib/data/ModPack.js
+++ b/packages/lib/data/ModPack.js
@@ -316,6 +316,40 @@ class ModPack {
 			propertyTreeBytes(this.toJSON().settings),
 		]);
 	}
+
+	/**
+	 * Fill in missing settings with their default values
+	 *
+	 * Uses the provided setting prototypes to add any missing mod settings
+	 * in the mod pack with the default value from the prototype.
+	 *
+	 * @param {Object<string, object>} settingPrototypes -
+	 *     Setting prototypes exported from the game.
+	 * @param {Logger} logger - Logger used to report warnings on.
+	 */
+	fillDefaultSettings(settingPrototypes, logger) {
+		const knownTypes = ["bool-setting", "int-setting", "double-setting", "string-setting"];
+		let prototypes = Object.entries(settingPrototypes)
+			.filter(([type, _]) => knownTypes.includes(type))
+			.flatMap(([_, settings]) => Object.values(settings))
+		;
+
+		for (let prototype of prototypes) {
+			if (!["startup", "runtime-global", "runtime-per-user"].includes(prototype.setting_type)) {
+				logger.warn(`Ignoring ${prototype.name} with unknown setting_type ${prototype.setting_type}`);
+				continue;
+			}
+			if (prototype.default_value === undefined) {
+				logger.warn(`Ignoring ${prototype.name} with missing default_value`);
+				continue;
+			}
+			if (this.settings[prototype.setting_type].get(prototype.name)) {
+				continue;
+			}
+
+			this.settings[prototype.setting_type].set(prototype.name, { value: prototype.default_value });
+		}
+	}
 }
 
 module.exports = ModPack;
diff --git a/packages/master/src/routes.js b/packages/master/src/routes.js
index 4f14fda3..4e48f1f5 100644
--- a/packages/master/src/routes.js
+++ b/packages/master/src/routes.js
@@ -220,12 +220,17 @@ async function uploadExport(req, res) {
 	];
 
 	let assets = {};
+	let settingPrototypes = {};
 	for (let filePath of exportFiles) {
 		let file = zip.file(filePath);
 		if (!file) {
 			continue;
 		}
 
+		if (filePath === "export/settings.json") {
+			settingPrototypes = JSON.parse(await file.async("text"));
+		}
+
 		let { name, ext } = path.posix.parse(filePath);
 		let hash = await libHash.hashStream(file.nodeStream());
 		assets[name] = `${name}.${hash}${ext}`;
@@ -233,6 +238,7 @@ async function uploadExport(req, res) {
 	}
 
 	modPack.exportManifest = new libData.ExportManifest({ assets });
+	modPack.fillDefaultSettings(settingPrototypes, logger);
 	res.app.locals.master.modPackUpdated(modPack);
 
 	res.sendStatus(200);
diff --git a/test/lib/data/ModPack.js b/test/lib/data/ModPack.js
index 15f94ee7..43cdc856 100644
--- a/test/lib/data/ModPack.js
+++ b/test/lib/data/ModPack.js
@@ -76,6 +76,99 @@ describe("lib/data/ModPack", function() {
 			assert.deepEqual(factorioMods.map(mod => mod.factorioVersion), sortedVersions);
 		});
 
+		describe(".fillDefaultSettings()", function() {
+			const prototypes = {
+				"bool-setting": {
+					"bool": {
+						name: "bool",
+						type: "bool-setting",
+						setting_type: "startup",
+						default_value: true,
+					},
+				},
+				"double-setting": {
+					"number": {
+						name: "number",
+						type: "double-setting",
+						setting_type: "runtime-global",
+						default_value: 123,
+					},
+				},
+				"string-setting": {
+					"string": {
+						name: "string",
+						type: "string-setting",
+						setting_type: "runtime-per-user",
+						default_value: "a string",
+					},
+				},
+			};
+			const mockLogger = { warn: () => {} };
+			it("should fill in defaults for settings", function() {
+				const pack = new ModPack();
+				pack.fillDefaultSettings(prototypes, mockLogger);
+				assert.deepEqual(pack.toJSON().settings, {
+					"startup": {
+						"bool": { "value": true },
+					},
+					"runtime-global": {
+						"number": { "value": 123 },
+					},
+					"runtime-per-user": {
+						"string": { "value": "a string" },
+					},
+				});
+			});
+			it("should not overwrite existing values", function() {
+				const pack = new ModPack({ settings: {
+					"startup": {
+						"bool": { "value": false },
+					},
+					"runtime-global": {
+						"number": { "value": 2 },
+					},
+					"runtime-per-user": {
+						"string": { "value": "spam" },
+					},
+				}});
+				pack.fillDefaultSettings(prototypes, mockLogger);
+				assert.deepEqual(pack.toJSON().settings, {
+					"startup": {
+						"bool": { "value": false },
+					},
+					"runtime-global": {
+						"number": { "value": 2 },
+					},
+					"runtime-per-user": {
+						"string": { "value": "spam" },
+					},
+				});
+			});
+			it("should ignore unknown setting_type and missing default_value", function() {
+				const pack = new ModPack();
+				pack.fillDefaultSettings({
+					"string-setting": {
+						"foo": {
+							name: "foo",
+							type: "string-setting",
+							setting_type: "magic-that-does-not-exist",
+							default_value: "a string",
+						},
+						"bar": {
+							name: "bar",
+							type: "string-setting",
+							setting_type: "startup",
+						},
+					},
+				}, mockLogger);
+				assert.deepEqual(pack.toJSON().settings, {
+					"startup": {},
+					"runtime-global": {},
+					"runtime-per-user": {},
+				});
+			});
+		});
+
 		describe(".fromModPackString()", function() {
 			it("should handle malformed strings", function() {
 				assert.throws(