KAI, a leading onsen ryokan brand by Hoshino Resorts, will open KAI Kusatsu in Kusatsu Onsen, Gunma Prefecture, on June 7, 2026. Centered under the concept of “A Nature-surrounded Onsen Ryokan Featuring a Private Tunnel," this property is located on a plateau that stretches across the foot of Mount Kusatsu-Shirane, proposing a novel accommodation experience that links the vibrancy of the hot spring town with the serenity at the ryokan through a guest-exclusive tunnel.
By seamlessly moving between these two distinct atmospheres, guests can enjoy a variety of attractions—including the famous waters of Kusatsu, the textile culture of Joshu, and a selection of multi-course kaiseki dinners—and discover a fresh way to experience the true essence of Kusatsu Onsen. 
Kusatsu Onsen Town
Boasting the highest natural discharge rate in Japan and selected as one of Japan’s Three Famous Hot Springs, Kusatsu Onsen is a historic hot spring destination long loved by the Japanese. Along the paths leading to the Yubatake (hot spring field), which dates back to the Edo period, numerous hot spring inns and shops stand side by side.
Experience tranquility at an onsen ryokan surrounded by the nature of Kusatsu-Shirane
Kusatsu Onsen has been loved since ancient times as a bustling hot spring town, long celebrated in song as having "a thousand houses of Kusatsu in the Edo style." Many shops and inns line the streets leading to the Yubatake (hot spring field), which has remained since the Edo period. In contrast to the hustle and bustle of the town, KAI Kusatsu is an extraordinary onsen ryokan situated on a quiet hill. The design of the property reflects the rock faces and vegetation unique to Kusatsu-Shirane, created by its highly acidic natural environment. Walking along the path through the grove, guests hear the gentle chirping of wild birds and feel the soft breeze amid the shifting dappled sunlight. During their stay, guests can wander through the buildings scattered across the garden, experiencing the healing power of serenity firsthand while surrounded closely by the rich, natural beauty of Kusatsu. 
An Onsen Ryokan with a Private Tunnel
A guest-exclusive tunnel serves as the gateway between these two distinct worlds. The passage allows guests to easily pair a relaxing stay at the ryokan with explorations of the local hot spring town—a dual experience that naturally lends itself to extended stays. 
■ Introducing "KAI Kusatsu Strolling Kit" to Elevate Local Exploration
During the stay, guests are encouraged to stroll through the hot spring town just beyond the tunnel dressed in yukatas, with a furoshiki bag in hand. To enrich the experience, the property offers the “KAI Kusatsu Town-Strolling Kit” for purchase. The kit includes a “Three-Bath Strolling Pass” for visiting Kusatsu Onsen’s three famous public baths, followed by a relaxing café break in the Ura-Kusatsu area.
The kit also includes an original guide titled “Recommendations for Strolling Kusatsu.” Curated by KAI staff, the guide highlights local spots and insider tips to offer an experience exclusive to KAI Kusatsu guests. Furthermore, you can easily venture out into the hot spring town via the tunnel for dining, allowing you to customize your stay with a wealth of options. This stay allows you to experience the essence of Kusatsu by immersing yourself in the history of the hot spring town and being healed by the serenity of the ryokan. 
Strolling through Kusatsu Onsen 
Strolling through Sainokawara Park
Features of KAI Kusatsu
1. Onsen: A Unique Bath-Hopping Experience Surrounded by Trees
The public bathhouse features two types of acidic hot spring water, allowing guests to fully experience the natural benefits of Kusatsu Onsen—an area boasting Japan's highest natural discharge rate. The first is the "Bandaiko Source," known for having some of the highest discharge volumes, temperatures, and acidity levels in the region. The second is the "Nishinogawara Source," a rare, milder acidic spring utilized by only a limited number of facilities. Inside, three separate bathtubs allow guests to experience these distinct water qualities at varying temperatures.
■ Spring Quality
Bandaiko Source: Acidic / Chloride / Sulfate hot spring
Nishinogawara Source: Acidic / Aluminum / Sulfate/Chloride hot spring
2. Public Space: Fireplace Lounge for Relaxing moments
Following a soak in the hot spring, guests can unwind in the adjacent Fireplace Lounge. The lounge is divided into four distinct zones, allowing visitors to find their preferred setting. At the center, a fireplace offers a warm space filled with the gentle sound of crackling wood. The lounge also functions as a Travel Library, where guests can relax at leisure while browsing a curated collection of books on the culture and nature of Gunma and Kusatsu.
3. Guest Rooms: "Silk Art Room"
Each KAI property features locally inspired signature rooms that reflect regional culture. At KAI Kusatsu, the “Silk Art Rooms” honor Gunma’s heritage of silk production. The centerpiece is a woven textile artwork depicting Kusatsu’s mountain landscape and rising hot spring steam, captured through delicate color gradations and fine silk threads. Designed by internationally acclaimed textile artist Reiko Sudo and crafted using traditional Kiryu weaving techniques, the rooms seamlessly blend artistry with comfort. Silk lampshades, custom cushions, and bed runners inspired by alpine flora create a soft, harmonious atmosphere throughout the space.
4. KAI Cultural Discovery: "Joshu Zaguri Silk Spinning"
As part of KAI’s signature hospitality, the property features “KAI Cultural Discovery" activities that connect guests with local heritage. Celebrating Gunma Prefecture’s history as a prominent textile hub, built on a legacy of sericulture, silk reeling, and weaving, KAI Kusatsu offers a “Joshu Zaguri Silk Spinning” experience. Using traditional handcraft techniques dating back to the Edo period, guests can operate a wooden zaguri (hand-reeling) machine to spin thread directly from silk cocoons, creating a custom tassel to take home as a souvenir.
The activity takes place in a dedicated pavilion inspired by the Tomioka Silk Mill, Japan's historic model mechanical silk-reeling factory. By experiencing the silk thread's strength and delicacy firsthand, guests gain a deeper appreciation for the fine textile artworks displayed throughout the resort. 
5. Dining: Flexible Options to Suit Every Itinerary
Guests at KAI Kusatsu can choose between two primary dining paths: the signature Joshu Hoden Kaiseki course or the Tejiko Nama-ko-uchi Soba Kaiseki served at Soba Kappo SAI. A new addition to the property, Soba Kappo SAI marks the KAI brand’s first venue open to day-use (non-staying) visitors. The restaurant features an à la carte menu alongside a variety of small plates to enjoy before the main soba dishes, offering a relaxed alternative to standard kaiseki dining. For further variety, guests can use the private tunnel to dine in the hot spring town, providing ample flexibility to customize their meals over a multi-night stay.
■ Option One: "Joshu Hoden Kaiseki"
At the main restaurant, guests can enjoy a kaiseki dinner in a semi-private dining space that highlights the seasonal flavors of Gunma. The multi-course Joshu Houden Kaiseki begins with an appetizer inspired by Kusatsu’s iconic Yubatake (hot spring field) and Yumomi *1 traditions. The signature main dish, Joshu Joyo Nabe, is a hot pot featuring whole simmered chicken thigh and local Gunma spinach served in a savory broth, offering a refined and comforting culinary experience.
*1:A traditional method to cool down Kusatsu's hot spring water to bathing temperature without diluting it with cold water.
■ Option Two: "Tejiko Nama-ko-uchi Soba Kaiseki" at Soba Kappo SAI
Soba Kappo SAI features 100% buckwheat juwari soba, crafted from locally sourced flour and paired with a rich, three-stage dipping broth brewed from bonito, kelp, and dried shiitake mushrooms. Situated along the edge of Nishinogawara Park, the restaurant’s interior offers a serene, refined dining environment framed by views of the surrounding greenery. Guests can complement their meal with a curated selection of local sake, regional craft beer, or natural wines for a sophisticated evening.
Property Overview
Property Name: KAI Kusatsu
Location: 464-690, Aaza Kusatsu Aza Shirane, Oaza Kusatsu, Kusatsu-machi, Agatsuma-gun, Gunma Prefecture
Phone: 050-3134-8092 (KAI Reservation Center)
Number of Rooms: 94 rooms (including 18 rooms with private open-air baths)
Rate: From 46,000 JPY per person (for 1 room shared by 2 people, inclusive of tax, service charges, breakfast, and dinner)
On-site Facilities: Front desk, Shop, Ski lockers, Dining venues, Bathhouse wing, Lounge wing, KAI Cultural Discovery wing, Courtyard, Dog run, Tunnel, Soba Kappo restaurant
Access: 60 minutes by car from Karuizawa Station, or 20 minutes by bus from Naganohara-Kusatsuguchi Station
Official Website: Hoshino Resorts KAI Kusatsu
Opening Date: June 7, 2026
Project Overview
Building Area: 3,701.72 m²
Total Floor Area: 11,892.88 m²
Site Area: 111,844 m²
Architectural Basic Design: Tatsuro Sasaki Architects & Associates
Landscape (LS) Basic Design: SEA BASS
Detailed Design & Construction: Maeda Corporation
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