A current flowing from nearshore waters back toward the open sea, gentle in some cases, violent in others. While riptides can contribute directly to good waves, they are frequently dangerous to swimmers and novice surfers. "Riptide" is something of a misnomer, as it's influenced but not caused by tide. A riptide is also known as rip current, or run-out, or (another misnomer) undertow.

Riptides are generally formed when waves break consistently over a shallow sandbar or reef, and the incoming lines of whitewater, unable to return seaward by the same route, instead briefly traverse the shore, then turn 90 degrees, almost always into a topographical low spot. Rips have been timed up to eight feet per second. Riptides adjacent to reefbreaks (like Hawaii's Sunset Beach) flow along permanent channels; riptides at beachbreaks will sometimes form temporary channels—which in turn can produce first-rate surf. Rips next to piers and jetties can also give a surfer a speeded-up route back to the lineup.

Riptides are usually about 30 to 50 yards wide, are more common during lower tides, and tend to dissipate not far beyond the surf zone. Because they produce chops and eddies as they flow outward, riptides are fairly easy to spot. Nonetheless, a majority of beachside lifeguard rescues worldwide—about 80%—are made when swimmers, and sometimes beginning surfers, wander into a riptide and are pulled out to sea. As the Los Angeles Times reported in 2005, riptides "kill more people each year than hurricanes, tornadoes, floods, or sharks." The paper went on to note that an average of 100 people in the US die each year in rips.

A "flash rip" occurs after a set of bigger-than-average waves suddenly creates strong but temporary current back toward the open ocean. If the beach is crowded and the set of waves are especially outsized, this type of rip can be especially dangerous, as was the case on "Black Sunday" at Bondi Beach, Australia, in 1938, when, over the course of 20 minutes, 35 rip-caught swimmers were pulled unconscious from the water, five of whom died.

Swimming or paddling directly against a riptide is nearly always futile; the easiest and most direct way out is to angle laterally across the current, into the nearby breaking surf—or to simply wait until the current dissipates, beyond the surf line.

Surf band Jon and the Nightriders recorded "Riptide" for Surf Beat '80, their debut album. British singer Robert Palmer titled ins 1985 LP Riptide. Australian singer-songwriter Vance Joy's "Riptide" (a different song from the Nightriders) was a global hit in 2013 and 2014. Riptide is also the name of a popular 1988-founded bodyboarding magazine, published out of Australia.