Each of the 150 prisons in England and Wales has been offered 100 envelopes to hand out to inmates.
Prisoners are seven times more likely than other people to take their own lives.
Joe Ferns, deputy director of service support at Samaritans, said: "We hope the free envelopes will enable people who have had problems with funds to contact us."
The charity has so far printed 7,000 envelopes at a cost of £1,500. It will pay second-class postage for each envelope used.
Prisoners who have already written to Samaritans Correspondence - the Scottish-based branch that deals with letters to the emotional support charity - will receive an envelope with their response.