Security staff and cleaners will go on strike for 48-hours in a row over pay and conditions.
The RMT said: "It involves safety-critical staff, so we expect to have an impact which will affect the running of the DLR."
General secretary of the union, Mick Cash, said: "It is about time that Interserve woke up and took note. The union will not sit back while core issues on pay and working conditions are left unresolved."
Transport for London and Interserve are yet to comment on the action.