It happened on the river between Magheramason and Carrigans.
The Northern Ireland Ambulance Service, the PSNI and the Irish coastguard were among those involved in the rescue.
The ambulance service said all 26 people have been taken to Altnagelvin Hospital where they are being treated for mild hypothermia.
The boat capsized at about 2pm.
Those rescued are understood to have been taking part in a charity event for the RSPCA.
SDLP Foyle assembly member Mark H Durkan tweeted: "Just been at scene of major incident response at River Foyle, think everyone out of water ok. Well done emergency teams!"