Evans, a 30-year-old has spent the last six seasons with Plymouth Albion, but only made the full-time switch to hooker for the last two campaigns, having previously operated in the back row and as a prop.
Evans has also represented the Navy in the annual match against the Army at Twickenham, and scored the winning try during the 2010 encounter.
He will combine his role as a Royal Marine sergeant with playing for the Exiles in their maiden Aviva Premiership campaign.
"It's a massive opportunity and I'm really looking forward to next season," Evans said.
"The Aviva Premiership is a massive challenge but we'll see how we get on. Unlike in the Championship we'll really have to hit the ground running, but hopefully with (coach) Lyn (Jones) in charge he'll guide us through."
The 26-year-old Runciman spent last season with Gloucester, having left Welsh after a three year stint at Old Deer Park.
Runciman, who also previously played for Worcester and was capped by England at age-grade level, made 15 appearances for the Cherry and Whites but will be hoping for more first-team opportunities with the Exiles.
"I'm very pleased to be back - it feels like I've never been away," said Runciman.
"It's a great place to play rugby, a great bunch of lads and I'm looking forward to playing under (head coach) Lyn (Jones).
"I watched all of London Welsh's televised games last season and kept my eye on their results. It was great to see the boys win the Championship the way they did. It was testament to the professionalism of the lads and the coaching staff at the club.
"Gloucester definitely benefited my game. I feel fitter than I've ever been and my game management's improved, and hopefully I can add that to the London Welsh squad."
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- Aviva Premiership