Ride-hailing firm Lyft Inc. has raised new capital that doubles its valuation from last year to $15.1 billion and gives it more firepower as bigger rival Uber Technologies Inc. tracks toward an initial public offering.

Lyft said it raised $600 million primarily from existing investors, about six months after raising $1.5 billion at an $11.5 billion valuation. It was valued at $7.5 billion in April 2017.

The...