diff --git a/CODEOWNERS b/CODEOWNERS
index 3aad92168b..96bd1e44d1 100644
--- a/CODEOWNERS
+++ b/CODEOWNERS
@@ -16,5 +16,6 @@ server/chat-plugins/rock-paper-scissors.ts @mia-pi-git
 server/chat-plugins/scavenger*.ts @xfix @sparkychildcharlie
 server/chat-plugins/the-studio.ts @KrisXV
 server/chat-plugins/trivia.ts @AnnikaCodes
+server/chat-plugins/username-prefixes.ts @AnnikaCodes
 server/modlog.ts @monsanto
 test/random-battles/* @AnnikaCodes
diff --git a/server/chat-plugins/username-prefixes.ts b/server/chat-plugins/username-prefixes.ts
new file mode 100644
index 0000000000..2e3ee44f21
--- /dev/null
+++ b/server/chat-plugins/username-prefixes.ts
@@ -0,0 +1,127 @@
+/**
+ * Code to manage username prefixes that force battles to be public or disable modchat.
+ * @author Annika
+ */
+
+import {FS} from '../../lib';
+
+const PREFIXES_FILE = 'config/chat-plugins/username-prefixes.json';
+
+export class PrefixManager {
+	constructor() {
+		// after a restart/newly using the plugin, load prefixes from config.js
+		if (!Chat.oldPlugins['username-prefixes']) this.refreshConfig(true);
+	}
+
+	save() {
+		FS(PREFIXES_FILE).writeUpdate(() => JSON.stringify(Config.forcedprefixes || {}));
+	}
+
+	refreshConfig(configJustLoaded = false) {
+		if (!Config.forcedprefixes) Config.forcedprefixes = {};
+		if (configJustLoaded) {
+			// if we just loaded the config file, ensure that all prefixes are IDs
+			if (Config.forcedprefixes.privacy) Config.forcedprefixes.privacy = Config.forcedprefixes.privacy.map(toID);
+			if (Config.forcedprefixes.modchat) Config.forcedprefixes.modchat = Config.forcedprefixes.modchat.map(toID);
+		}
+
+		let data: AnyObject;
+		try {
+			data = JSON.parse(FS(PREFIXES_FILE).readSync());
+		} catch (e) {
+			if (e.code !== 'ENOENT') throw e;
+			return;
+		}
+		for (const [type, prefixes] of Object.values(data)) {
+			if (!Config.forcedprefixes[type]) Config.forcedprefixes[type] = [];
+			for (const prefix of prefixes) {
+				if (Config.forcedprefixes[type].includes(prefix)) continue;
+				Config.forcedprefixes[type].push(prefix);
+			}
+		}
+	}
+
+	addPrefix(prefix: ID, type: 'privacy' | 'modchat') {
+		if (!Config.forcedprefixes[type]) Config.forcedprefixes[type] = [];
+		if (Config.forcedprefixes[type].includes(prefix)) {
+			throw new Chat.ErrorMessage(`Username prefix '${prefix}' is already configured to force ${type}.`);
+		}
+
+		Config.forcedprefixes[type].push(prefix);
+		this.save();
+	}
+
+	removePrefix(prefix: ID, type: 'privacy' | 'modchat') {
+		if (!Config.forcedprefixes[type]?.includes(prefix)) {
+			throw new Chat.ErrorMessage(`Username prefix '${prefix}' is not configured to force ${type}!`);
+		}
+
+		Config.forcedprefixes[type] = Config.forcedprefixes[type].filter((curPrefix: ID) => curPrefix !== prefix);
+		this.save();
+	}
+
+	validateType(type: string) {
+		if (type !== 'privacy' && type !== 'modchat') {
+			throw new Chat.ErrorMessage(`'${type}' is not a valid type of forced prefix. Valid types are 'privacy' and 'modchat'.`);
+		}
+		return type;
+	}
+}
+
+export const prefixManager = new PrefixManager();
+
+export const commands: Chat.ChatCommands = {
+	forcedprefix: 'usernameprefix',
+	forcedprefixes: 'usernameprefix',
+	usernameprefixes: 'usernameprefix',
+	usernameprefix: {
+		help: '',
+		''() {
+			this.parse(`/help forcedprefix`);
+		},
+
+		delete: 'add',
+		remove: 'add',
+		add(target, room, user, connection, cmd) {
+			this.checkCan('rangeban');
+
+			const isAdding = cmd.includes('add');
+
+			const [prefix, type] = target.split(',').map(toID);
+			if (!prefix || !type) return this.parse(`/help usernameprefix`);
+			if (prefix.length > 18) {
+				throw new Chat.ErrorMessage(`Specified prefix '${prefix}' is longer than the maximum user ID length.`);
+			}
+
+			if (isAdding) {
+				prefixManager.addPrefix(prefix, prefixManager.validateType(type));
+			} else {
+				prefixManager.removePrefix(prefix, prefixManager.validateType(type));
+			}
+
+			this.globalModlog(`FORCEDPREFIX ${isAdding ? 'ADD' : 'REMOVE'}`, null, `'${prefix}' ${isAdding ? 'to' : 'from'} ${type}`);
+			this.addGlobalModAction(`${user.name} set the username prefix ${prefix} to${isAdding ? '' : ' no longer'} disable ${type}.`);
+		},
+
+		view(target) {
+			this.checkCan('rangeban');
+
+			const types = target ? [prefixManager.validateType(toID(target))] : ['privacy', 'modchat'];
+
+			return this.sendReplyBox(types.map(type => {
+				const info = Config.forcedprefixes[type].length ?
+					`<code>${Config.forcedprefixes[type].join('</code>, <code>')}</code>` : `none`;
+				return `Username prefixes that disable <strong>${type}</strong>: ${info}.`;
+			}).join(`<br />`));
+		},
+	},
+	usernameprefixhelp() {
+		return this.sendReplyBox(
+			`<code>/usernameprefix add [prefix], [type]</code>: Sets the username prefix [prefix] to disable privacy or modchat on battles where at least one player has the prefix.<br />` +
+			`<code>/usernameprefix remove [prefix], [type]</code>: Removes a prefix configuration.<br />` +
+			`<code>/usernameprefix view [optional type]</code>: Displays the currently configured username prefixes.<br />` +
+			`Valid types are <code>privacy</code> (which forces battles to take place in public rooms) and <code>modchat</code> (which prevents players from setting moderated chat).<br />` +
+			`Requires: &`
+		);
+	},
+};
diff --git a/server/index.ts b/server/index.ts
index eaa0a2e30a..e6a73d7fd3 100644
--- a/server/index.ts
+++ b/server/index.ts
@@ -90,6 +90,9 @@ if (Config.watchconfig) {
 	FS(require.resolve('../config/config')).onModify(() => {
 		try {
 			global.Config = ConfigLoader.load(true);
+			// ensure that battle prefixes configured via the chat plugin are not overwritten
+			// by battle prefixes manually specified in config.js
+			Chat.plugins['username-prefixes']?.prefixManager.refreshConfig(true);
 			Monitor.notice('Reloaded ../config/config.js');
 		} catch (e) {
 			Monitor.adminlog("Error reloading ../config/config.js: " + e.stack);
diff --git a/test/server/chat-plugins/username-prefixes.js b/test/server/chat-plugins/username-prefixes.js
new file mode 100644
index 0000000000..58f2e1ced5
--- /dev/null
+++ b/test/server/chat-plugins/username-prefixes.js
@@ -0,0 +1,44 @@
+/**
+ * Tests for the username-prefixes chat plugin.
+ * @author Annika
+ */
+'use strict';
+
+const assert = require('assert').strict;
+const {PrefixManager} = require('../../../.server-dist/chat-plugins/username-prefixes');
+
+describe('PrefixManager', () => {
+	beforeEach(() => {
+		this.prefixManager = new PrefixManager();
+		Config.forcedprefixes = {privacy: [], modchat: []};
+	});
+
+	it('Config.forcedprefixes should reflect prefix additions and removals', () => {
+		this.prefixManager.addPrefix('forcedpublic', 'privacy');
+		this.prefixManager.addPrefix('nomodchat', 'modchat');
+
+		assert(Config.forcedprefixes.privacy.includes('forcedpublic'));
+		assert(Config.forcedprefixes.modchat.includes('nomodchat'));
+
+		this.prefixManager.removePrefix('forcedpublic', 'privacy');
+		this.prefixManager.removePrefix('nomodchat', 'modchat');
+
+		assert(!Config.forcedprefixes.privacy.includes('forcedpublic'));
+		assert(!Config.forcedprefixes.modchat.includes('nomodchat'));
+	});
+
+	it('should not overwrite manually specified prefixes', () => {
+		Config.forcedprefixes.modchat = ['manual'];
+		this.prefixManager.addPrefix('nomodchat', 'modchat');
+
+		assert.deepEqual(Config.forcedprefixes.modchat, ['manual', 'nomodchat']);
+	});
+
+	it('should correctly validate prefix types', () => {
+		assert.doesNotThrow(() => this.prefixManager.validateType('privacy'));
+		assert.doesNotThrow(() => this.prefixManager.validateType('modchat'));
+
+		assert.throws(() => this.prefixManager.validateType('gibberish'));
+		assert.throws(() => this.prefixManager.validateType(''));
+	});
+});