A modded PSP can play ad hoc games online through the same relay servers as PPSSPP, using Kethen's aemu plugin, a maintained fork of the Pro Online plugin. Relay mode is aemu's default, so you don't need port forwarding, UPnP or a VPN. You can play with PPSSPP and PS Vita players on the same server.

What you need

A PSP 2000/3000 or PSP Go on 6.61 custom firmware. ARK-4 is recommended; 6.61 PRO-C still works for now, but future aemu releases may need ARK-4. A PSP 1000 runs more games than before, but some games that work on the slim models may not work on it.
A 2.4 GHz Wi-Fi network your PSP can connect to. Avoid special characters in the network name (SSID).
The latest aemu release from GitHub.

Set up a PSP 2000/3000

In the XMB, go to Settings > Network Settings > Infrastructure Mode and set up your Wi-Fi connection. Write down its SSID, and test it with Test Connection from the Triangle menu.
In Settings > Power Save Settings, set WLAN Power Save to Off.
In Settings > System Settings, set UMD Cache to Off.
Press Select in the XMB to open the recovery menu and change these settings:
ARK-4 (Custom Firmware Settings): CPU Clock in Game = OverClock, Use Extra Memory = Auto, Memory Stick Speedup = Off, Inferno Cache = Off.
PRO: Advanced > Memory Stick Speedup = None, Advanced > Inferno & NP9660 Use ISO Cache = Disabled, Advanced > Force High Memory Layout = Disabled, CPU Speed > Game CPU/BUS = 333/166.
Unzip aemu and copy the kd and seplugins folders from dist_660_release to the root of your memory stick (ms0:/).
Turn on the plugin by adding this line, if it isn't there already:
ARK-4: game, ms0:/seplugins/atpro.prx, on in ms0:/seplugins/plugins.txt
PRO: ms0:/seplugins/atpro.prx 1 in ms0:/seplugins/game.txt
Open ms0:/seplugins/hotspot.txt and type the SSID from step 1.
Open ms0:/seplugins/server.txt and type the relay server address. The default is socom.cc; change it to the server your group plays on, for example psp.mgn.pub. See the full PSP Relay server list.
Leave ms0:/seplugins/port_offset.txt at 10000, which matches PPSSPP's default.
Delete ms0:/kd/pspnet_miniupnc.prx if it exists.
If games crash, turn off your other game plugins; some conflict with aemu.

PSP Go

Follow the same steps, but copy kd and seplugins to both the memory stick (ms0:/) and the internal storage (ef0:/), and repeat steps 6 to 10 on ef0:/ as well. For example, add game, ef0:/seplugins/atpro.prx, on to ef0:/seplugins/plugins.txt on ARK-4, or ef0:/seplugins/atpro.prx 1 to ef0:/seplugins/game.txt on PRO.

Play

Start the game and open the aemu overlay to check that it loaded: L + R + Select on ARK-4, or the Home button on other custom firmware. If it doesn't appear, go back over the steps.
Go into the game's ad hoc multiplayer mode and create or join a room. The overlay also has a chat while you're in a group.

Tips

Everyone needs the same relay server, port offset 10000, and the same game region and version.
For lag or random disconnects, pick the server closest to you and use a quiet 2.4 GHz channel. The PSP's 802.11b Wi-Fi is easily disturbed, so a dedicated channel helps.
Still stuck? Copy kd and seplugins from dist_660_debug instead and check ms0:/atpro.log.
Game-specific notes are in aemu's known issues and tips. For example, in Star Wars: The Force Unleashed the console should host when playing with PPSSPP.

Video guides

PSP X PPSSPP | MULTIPLAYER ONLINE no PSP em 2026 | Guia completo AEMU by Gio Estamina Lite (April 2026, in Portuguese). A full aemu setup on ARK-4 and PRO with a Brazilian relay server. A viewer wrote: "Deu certo aqui no meu PSP go🎉🎉" ("It worked on my PSP Go").
How to play Ad-hoc multiplayer between a real PSP / Vita and PPSSPP emulator by PSP Mav (December 2023). This one is older: it uses the original Pro Online plugin on a single Wi-Fi network, but installing the plugin works much the same way. Use the current aemu files and the relay settings from this guide. A viewer wrote: "Ive been searching for this from the past few months so i can play with my friend with ppsspp and me with psp.. Thanks!!"

PSP Relay on a real PSP