Since the late 1990s, Japan has witnessed a substantial increase of partial mergers where two or more firms spin off whole operations in the same business and combine them into a joint venture (JV). This paper provides the first academic evidence on this phenomenon. I find that partial mergers normally occur as a response to negative economic shocks by firms that are larger and more diversified than firms in total mergers. An event study identifies positive and significant returns to partial merger announcements. Unlike total mergers whose value accrues mostly to the shareholders of small (acquired) firms, large and small firms in partial mergers receive comparable returns, which are particularly large to firms forming an equally owned JV. This study also finds that partial mergers are often ex post transformed, with equity sale between partners being the main source of change. © 2010 Elsevier B.V.