Currently the offerings are available on Comcast's online hub for content, XfinityTV.com. Showtime also plans to launch website ShowtimeAnytime.com, where subscribers of other cable, satellite and telephone companies will be able to watch the shows later this year.
The service is similar to Time Warner Inc.'s HBO Go, an online service that gives paying subscribers access to HBO programming online. Both are meant to add value to subscriptions while keeping online content behind a so-called "pay wall" that ensures revenue is coming in.
The online access to Showtime was part of a 10-year deal CBS and Comcast announced in August.
Tom Christie, the executive vice president of affiliate sales for Showtime, said the new service was meant to keep Showtime relevant with consumers and "maintain and build the subscription business" with distributors.
Showtime expects to have about 18.5 million subscribers by the end of the third quarter, up about 1 million from a year earlier, Christie said.
CBS reports its quarterly earnings on Nov. 4.