SO PR will continue to operate as a standalone own brand, with Claire Sawkins as a major shareholder and managing director.
The Leeds-based agency specialises in consumer and b2b PR, and its client roster includes Tomy, Weber and Birley Manufacturing.
MUSE Group intends to increase its acquisitions of other comms companies over the next five years.
John Williams, chairman of MUSE, said: "We continue to look for partner businesses with like-minded people in the communications sector to acquire, merge with and/or form strategic alliances.
"Our goal is to attract some of the best talent in the UK to work on a raft of consumer and business-to-business brands at the cutting edge of communication."