Companies jockeying for dominant positions in Asian markets, beware: Regulators in China and Japan are launching new efforts to strengthen their antitrust policies.

The two countries are taking starkly different approaches. Japan wants to bring its competition laws up to Western standards, and to that end the Japan Fair Trade Commission has proposed revising the major overhaul it implemented less than two years ago. The latest reforms would broaden the scope of who can be fined by the JFTC for violating the rules, a move that could work in foreign companies’ favor.

China’s efforts, however, are much less clear. The ...