Uber Technologies Inc. has reached an agreement in principle to sell most of its Southeast Asia operations to local rival Grab Inc., ending a costly fight for market share in the fast-growing region, according to people familiar with the matter.

In exchange for its operations in Southeast Asia, Uber would gain a roughly 30% stake in Grab, these people said. The two companies are still hashing out the final terms of the pact, the people said, cautioning any deal would be subject to regulatory scrutiny. One of the people said...