Novo Nordisk on Thursday launched an unsolicited bid valued at up to $9 billion for obesity-drug developer Metsera, hoping to top the deal Pfizer reached last month and teeing up a potential legal battle for the biotech company.

Denmark's Novo Nordisk launched a rival bid for U.S. obesity biotech firm Metsera on Thursday, hijacking an offer from Pfizer as drugmakers jostle for advantage in the fast-growing and competitive wei...