LONDON — Johnson & Johnson, the world’s largest health care company, has moved to bolster its roster of treatments for rare diseases, announcing a $30 billion deal on Thursday to acquire Actelion, a Swiss biotechnology firm.

After months of stop-start negotiations — Johnson & Johnson walked away from discussions in mid-December, only for the companies to restart exclusive talks about a deal a few days later — the deal would add Actelion’s treatments for pulmonary arterial hypertension, or high blood pressure in the lungs, to the American giant’s stable.

As part of the deal, Actelion would spin off its drug discovery operations and early-stage clinical development assets into a new Swiss biopharmaceutical company, which would be listed in Switzerland.

“We believe this transaction offers compelling value to both Johnson & Johnson and Actelion shareholders,” Alex Gorsky, the chairman and chief executive of Johnson & Johnson, said in a news release.