Viacom Inc.’s BET Networks is preparing to launch a streaming-video service aimed at African-American audiences, according to people familiar with the plans, as the media giant pursues new business lines to counter the decline of its traditional pay-TV operation.

The service, BET+, is slated to launch in the fall. Pricing isn’t final, but Viacom executives expect it to cost more than the company’s Comedy Central Now service, which is $3.99 a month, one of the people said.

BET+...