Comcast, the country’s largest cable operator, announced Monday that it is rolling out its own version of an online-only streaming video app, on the heels of similar offerings from competitors such as HBO.

The app, called Stream, comes with a $15-a-month “skinny bundle” that lets Comcast Internet subscribers watch live television — as well as HBO and other on-demand content — on their laptops, tablets and phones.

It won’t be an exact duplication of a typical TV experience since there will be some content customers won’t be able to watch, including live broadcast television. But Comcast is betting that the service will appeal to folks who want to watch TV without being tied to the living-room couch.

The economics of the app appear to favor cord-cutters. For example, Stream includes HBO (along with a slew of other programming) making it unnecessary for Comcast customers to buy HBO’s own standalone app, HBO Now, which is also priced at $15.

If it takes off, Stream could help Comcast keep some of the revenues it loses when customers downgrade to Internet-only plans and then turn to online content providers such as Hulu and Netflix.

Comcast says it will launch Stream in Boston by “the end of the summer,” with Chicago, Seattle and then the rest of the country following soon after.

The cable giant’s decision to hop on board the streaming bandwagon is a sign of how mainstream online video has become. Although cable and satellite TV providers worry about the impact of cord-cutting on their bottom lines — potentially undercutting the business model that has sustained the industry for decades — the pace of new streaming products has picked up dramatically in recent months.

Television, like the rest of our entertainment, is migrating to the Web. And for Comcast to acknowledge that is a huge deal.