Teneo Blue Rubicon and krow will work on the strategic and creative direction for the charity’s Respect the Water campaign, which is designed to raise awareness of the dangers around water.
The agencies pitched for the business together.
RNLI’s media planning and buying is handled by OMD UK.
Ross MacLeod, community safety marketing manager at the RNLI, said: "The agencies bring a wealth of behaviour change and marketing expertise, which we really believe will help us to save more lives at sea with this exciting campaign."
Cancer Research UK, which had revenues of £634.9m in 2015, is the largest.