The Rail Maritime and Transport Union (RMT) has said its members on Docklands Light Railway (DLR) will walk out for three days on 23 June over a pay row.
RMT general secretary Bob Crow said: "Our members will not be bullied by management into taking on more work without being properly compensated."
The DLR members work for Serco, which called their demand "opportunistic".
The stoppage will coincide with a 48-hour strike called by some London Underground maintenance workers.
Mr Crow added: "RMT has made every effort to reach a negotiated settlement on this issue, but all of our proposals have been thrown back in our face.
"That is no way to run a key element of London's transport system."
David Godley, managing director of Serco Docklands, said: "This threat of industrial action is an opportunistic demand for cash for employees who have not had any change to job descriptions or terms or conditions of their employment.
"We have offered further talks with the RMT and are awaiting a reply."
Tube Lines maintenance workers on London Underground are due to stage two 48-hour strikes from 23 June and 14 July in a separate row over pay, jobs and conditions.