Bonnets Are the Winter Hat of the Moment. For Adults.

Each winter, it’s often the case that a specific cold-weather accessory — a rainbow-check scarf, for example — comes to define the season. This year that item appears to be a knit hat that could be described as a baby bonnet for adults. It also evokes a balaclava, leading some sellers to christen it the “balabonnet.”

The accessory, which has been embraced by a certain set of fashionable women, comes in various interpretations that range from girlie to monastic. Many styles can be tied under the chin to create a streamlined egg shape, and some have longer straps that can be wrapped around the neck like a scarf for a fully snooded look.

There are bonnets embellished with sterling-silver rings, like the version by Gemsun, a brand in New York City. Mimi Wade, a label in Los Angeles, makes a cutesy style with pointed cat ears. The hats are also sold at mall chains like Free People and Hot Topic; the latter offers a bonnet covered in tiny pink bows.

A $210 version by Pien Studios, a four-year-old label in Amsterdam, has emerged as one of the most covetable. The fuzzy hat, which the brand calls a balaclava, is made of a blend of mohair, merino wool and silk and has skinny, scarflike ties. Produced in a handful of colors, it is sold at trendy boutiques across the world, including Amomento in Seoul, Esmeralda Serviced Department in Tokyo and Carmen in Amsterdam.

Grace Hwang, an advertising creative director in Brooklyn, bought a Pien Studios hat last year at Tangerine, a multibrand store in Williamsburg. Ms. Hwang, 33, said she had noticed women in New York City wearing bonnets of various designs, and called her Pien Studios version the most versatile winter accessory that she owns. (She prefers to call it a hat-scarf, not a bonnet.)