Bristol-Myers Squibb Co. agreed to buy rival Celgene Corp. in a deal valued at about $74 billion, combining two leading sellers of cancer drugs and potentially signaling the return of dealmaking to the pharmaceutical industry.

Overall, the merged company will have nine products with more than $1 billion each in annual sales—most notably Celgene’s multiple myeloma drug Revlimid and Bristol’s lung-cancer treatment Opdivo.

Under...