GMB union members will join hundreds of thousands of NHS staff striking for four hours across England and Northern Ireland in a row over pay on 13 October.
The trade union, which balloted 22,000 of its members in the NHS in England and Northern Ireland, said more than seven out of ten (78%) of people voted in favour of strike action and 91% voted in favour of action short of a strike.
"GMB members have spoken loud and clear. They are prepared to take industrial action in the NHS to fight for fairness in pay," said Brian Strutton, GMB national secretary for public services.
"Nobody in the NHS wants to go on strike, but the anger and frustration of the workforce with the cavalier treatment by government and employers towards them has spilled over into industrial unrest."
But Strutton stressed that the GMB is open to talks with government and employers.
The action is part of an ongoing pay row between the government and the unions after the Health Secretary Jeremy Hunt rejected the NHS Pay Review Body's recommendation to give all of the NHS' staff a pay rise for 2014/15.
The move meant 600,000 workers, out of 1.3 million of the NHS' workforce, missed out on a wage hike.
Hunt argued at the time that the workers already got a pay rise worth on average 3% per year.
Unison, which has around 300,000 health members in England, has also said its members will be striking on 13 October.
The union said 68% voted yes for industrial action and 88% for action "short of strike action" in a ballot earlier this month.
A Department of Health spokesperson said: "We are disappointed that Unison is planning industrial action and has rejected our proposals to give NHS staff at least 1% additional pay this year and at least a further 1% next year."