Shareholders of Ra Pharma will receive $48 for each share held, a joint statement by the two companies said Thursday.
The transaction has been unanimously approved by the boards of both companies, the statement said, adding the deal is expected to be completed in the first quarter next year.
UCB will finance the acquisition via a combination of cash and loans.
Ra Pharma is a biopharmaceutical company focused on rare diseases. UCB is also a biopharmaceutical company that focuses on treatment of immune and central nervous system diseases.
Shares of RA Pharma gained more than 100%, or $22.96 a share, to $45.65 in trading on the Nasdaq Stock Market. They ended the day Wednesday at $22.70.