It has called off a three day walkout from 22 December and instead announced 48 hours of industrial action from 19 December, following consultation with reps and members.
RMT's general secretary Mick Cash said: "Our guards on Southern are in daily contact with passengers and the support for our fight to defend safety remains phenomenal."
He said that it is due to the "daily feedback from passengers" that the union has taken the decision to alter the dates for action over Christmas. "That's the difference between us and Southern. We listen, they don't,” Cash said.
Southern passengers have suffered months of disruption as ongoing disputes over the role of guards has led to industrial action.
Train drivers' union Aslef is also calling strikes over the festive period: on Tuesday 13 and Wednesday 14 December, Friday 16 December and Monday 9 to Saturday 14 January. In addition, drivers will not work any non-contractual work from Tuesday 6 December.
A Southern spokesperson said: "We welcome the decision by the RMT to cancel these dates immediately before Christmas but by announcing new dates in the same week they will still be disrupting passengers’ travel plans at this busy time."
They added that the new strike dates were "clearly coordinated" to follow the Aslef strikes and "cause maximum disruption".
Southern strike dates:
- Tuesday 6 to Thursday 8 December (RMT conductors’ strike)
- Tuesday 13 and Wednesday 14 December (Aslef & RMT drivers’ strike)
- Friday 16 December (Aslef & RMT drivers’ strike)
- Monday 19 December and Tuesday 20 December (RMT conductors' strike)
- Saturday 31 December to Monday 2 January (RMT conductors’ strike)
- Monday 9 to Saturday 14 January (Aslef & RMT drivers’ strike)
Transport secretary Chris Grayling has called the proposed industrial action on Southern "politically motivated" and "designed to create maximum disruption".
In a letter to Simon Kirby, MP for Brighton Kemptown, Grayling said: "We cannot allow the unions to dictate how or whether the railways should be modernised or not. They are seeking the right to determine when a train should be cancelled to inconvenience passengers, even in circumstances when this doesn't need to be the case."
He also said there were plans for a compensation package for Southern passengers in the works.
Cash said in response: "If anyone's politicising this dispute on Southern rail, it's Chris Grayling and his government."