Welded (or bonded) seams use heat or glue to join fabric panels together instead of stitching them and then covering with waterproof tape. Because there are no needle holes from stitching, there's inherently less opportunity for water to sneak in. The result is excellent waterproofing with a noticeably cleaner, sleeker look—no bulky tape strips visible inside the jacket.
Compared to fully taped seams, welded seams also reduce weight and bulk, which is why they're popular on premium technical jackets where packability matters. The trade-off is cost—these jackets tend to be expensive—and long-term durability can vary by manufacturer. Some bonded seams hold up season after season, while others may start to break down with heavy wear.
If you're investing in a high-end jacket for backcountry touring or want top-tier storm protection with a minimalist aesthetic, welded seams are a solid upgrade. For most resort riders, fully or critically taped seams will keep you dry at a friendlier price.