The Stockholm-based company, which launched in 2008 and arrived in the United States in 2011, said Wednesday that its monthly active users now total 40 million.
That means three quarters of its customers opt for the free, ad-supported version, rather than paying $10 a month to avoid commercials and get unlimited mobile access.
The new numbers represent a big jump from last year, when the company said it had 6 million subscribers and 24 million overall active users. The service is available in 56 countries.
The milestone comes as the technology and music industries wait for Apple to confirm reports that it will buy fashionable headphone maker Beats Electronics for $3.2 billion.
Beats, founded by Jimmy Iovine and Andre Young (better known as hip-hop star Dr. Dre) in 2008, in January launched its own streaming music service called Beats Music.
Beats Music, which does not offer a free version, has not released user statistics, but it is said to have brought in around 200,000 subscribers. San Fransisco-based Rdio hasn't put out its numbers either and is thought to be well behind Spotify, though it recently said it was now available in 60 countries.