Grand Seiko Debuts New 40mm Spring Drive U.F.A. Ushio 300 Diver

Grand Seiko introduced its revolutionary Spring Drive Caliber 9RB2 just over a year ago, at Watches & Wonders 2025 in Geneva. It was notable as the first movement from the Japanese high-horology house to achieve its new “U.F.A.” designation, whose initials stand for “Ultra Fine Accuracy.” The name was not an idle boast, as Caliber 9RB2, installed inside two dressy editions from the brand’s Evolution 9 collection, could claim an annual (not daily, not weekly) accuracy of +/- 20 seconds, making it very likely the most accurate wristwatch movement powered by a mainspring. Today, as Grand Seiko marks its fifth year exhibiting at the world’s most important watch-industry showcase, it has introduced the second generation of its Spring Drive U.F.A. movement and placed it inside a pair of much sportier, albeit still luxurious, models in the Evolution 9 series, the Ushio 300 Diver.

The watch is offered in two colorways, the Ref. SLGB023, with a blue dial, and Ref. SLGB025, with a green dial. Both models — like the SLGB003 that preceded them last year — feature a case made of Grand Seiko’s High Intensity Titanium, an alloy that is about 30 percent lighter than stainless steel and bears a more lustrous surface than one usually finds in other titaniums. Perhaps most newsworthy in this area is the fact that these cases measure just 40.8mm in diameter. This not only makes these new models substantially smaller than the existing Evolution 9 Ushio Diver, at its rather hefty 43.8mm size, but also makes them the smallest dive watches currently produced by Grand Seiko. 

The case is 12.9mm thick, with a screw-down crown and caseback that help ensure the 300-meter water resistance alluded to in the watch’s lengthy model name.  The sapphire crystal over the dial is treated on both its surfaces with antireflective material. The 120-click unidirectional bezel has a dive-scale insert made of ceramic, in a color matching the dial’s, that lends it a high level of resistance to scratches and corrosion.

Grand Seiko has given the dials the eye-catching Ushio textured effect that it debuted on the original model back in 2023, while also expanding the color palette for that particular treatment. The name means “tide” in Japanese, and entices the eye with its cascading, undulating wave texture that takes its cues from roiling ocean currents off the coast of Japan, a key inspiration in the nation’s art and culture. The blue dial of the SLGB023 evokes the vast expanse of deep seas that surround the Japanese archipelago, while the green dial of the SLGB025 calls to mind the shallower waters closer to shore. Both dials have a gradient effect, with brighter colors in the center giving way to darker hues at the outer edge, to echo the effect of light penetrating deep into water. 

The aesthetic details of the Evolution 9 family, which is descended from a classic Grand Seiko model from 1967, are all present on the textured dials. The rectangular hour indexes are diamond-cut and multifaceted, while the distinctive handset combines a trapezoidal hour hand with a broad arrow minute hand, plus a thin seconds hand with a bubble counterweight. The analog power reserve indicator at 8 o’clock conveys the state of the movement’s 72-hour power reserve. In keeping with Grand Seiko tradition, the hands and markers are coated in LumiBrite, the company’s proprietary luminous substance, which imparts to them a bright, vivid glow in low light. 

Behind the solid caseback, the aforementioned Spring Drive Caliber 9RB1 does its duty, its amazing accuracy of +/- 20 seconds per year enabled by a clever construction developed by the craftspeople at the company’s Shinshu Watch Studio in Japan’s mountainous Nagano Prefecture. The movement’s quartz crystal oscillator and its integrated circuit (IC) are vacuum-sealed to minimize the ill effects of external factors, like temperature, humidity, static electricity, and light, on their performance. (For those unfamiliar with how a Spring Drive movement works, let me refer you to this article.) 

Grand Seiko has left nothing to chance in securing the unprecedented accuracy of its movement, measuring the oscillator’s frequency at multiple temperatures and programming the resulting data into the IC to ensure proper thermo-compensation under any circumstances. Even the small divergences in accuracy that might occur after long periods of time can be easily adjusted during after-sales service by a dedicated regulation switch built into the movement. 

The bracelet of the Grand Seiko Spring Drive Ushio 300 Diver, which the Japanese manufacture touts as striking “the perfect balance between function and beauty,” is on the same High-Intensity Titanium as the case, with the same elite level of multifaceted finishing. Debuting on this version is a newly developed locking extension clasp with a three-step micro-adjustment system that allows for a total of 24mm of adjustment — the standard 6mm plus an additional 18mm, making it perfect for those stalwarts who actually want to wear this watch over their gloves on an actual dive. Available in June 2026, the watch will carry an MSRP of $12,400 in both its colorways. You can learn more at grand-seiko.com