The US firm already dominates the action-cam market, having sold more than $363m (Â£233m) worth of goods over the first three months of the year.
The Hero4 Session benefits from being waterproof without needing a special case, but cannot go as deep as models fitted with separate housings.
One expert said the firm risked having a "confusing" choice of products.
GoPro is marketing the new camera as being 50% smaller and 40% lighter than other Hero4 models.
It is:
Although the new camcorder may appear to have lower specifications than the firm's other options, GoPro suggests that its compact form coupled with a new dual-microphone system - designed to better handle wind noise - should appeal to customers intending to use it on a regular basis.
"We challenged ourselves to produce the smallest, lightest, most convenient GoPro possible," said the firm's founder Nicholas Woodman.
"Sound quality is as important as image quality and we're very excited about the stunning audio advancements we've made."
Costing Â£330, the Hero4 Session is only outpriced by the company's top-end Hero4 Black.
One tech industry watcher said it would probably have "niche" appeal, but added that GoPro's current line-up risked bewildering consumers.
"GoPro owns so much of the market that this is about trying to scoop up whatever is left," said James Trew, deputy managing editor of the Engadget news site.
"It used to have just three cameras - good, better and best - and you paid for the one you thought was your level.
"Now, in an effort to grow, I think there is a risk that it will start confusing customers, especially when the price of the Session is very similar to what are, on paper, better products.
"But there will be some people it will call out to - for instance, people who might think this looks better on their helmet."
GoPro faces a growing number of rivals, with EE, Xiaomi, Garmin and HTC among companies to have launched their own action cameras over recent months.
The California-based firm notes, however, that it currently accounts for all of the US's top five best-selling products in the digital camera/camcorder category, according to data published by market research firm NPD.
Even so, Mr Trew suggested that executives might feel they need to prove they are not complacent.
"GoPro went public last year so they have a whole new set of pressures to deal with," he explained.
"I think that's part of why we're now seeing more different stuff - including 360-degree camera mounts, virtual reality systems and broadcast technologies - to illustrate that it is staying ahead of the game in the face of increased competition."
GoPro has announced a new camera that is smaller and lighter than its earlier models.