Hill & Knowlton, which was formerly retained on a similar brief for two years, will handle two further media relations projects for the RFU.
RFU comms head Richard Prescott said Lexis was appointed after a five-way pitch.
Lexis aims to boost awareness of the sport and its top players, through media relations in the consumer, lifestyle and youth media.
Prescott said the fact H&K has been kept on for project work meant he was not dissatisfied with that agency's work but it was time for a 'different approach and different ideas'.
The rugby season is just four games old, but Prescott said attendances in the Zurich Premiership were up 13 per cent on this time last year.
In terms of grassroots participation, Prescott said the number of children and adults playing rugby had begun to increase in the past 18 months.
H&K is assisting with the RFU's press conference to launch its solo bid to host the 2007 World Cup. A decision on which country will host the event will be taken in two months' time.
H&K will also handle media work for the RFU's annual financial results in November.
In May the RFU appointed Dave Barton to the newly created post of regional media manager. He appointed four part-time regional press officers to his team two months ago.
The RFU has retained PPS Group for lobbying work for just over one year.