Russia and China Conduct First-Ever Joint Submarine Patrol in Sea of Japan, Alarming U.S., Japan, and South Korea

 Russian and Chinese submarines have conducted a coordinated joint patrol in the Sea of Japan, shifting their naval cooperation from the visible surface to the opaque depths of undersea warfare. This unprecedented maneuver followed Maritime Interaction/Joint Sea 2025, a bilateral exercise staged in Vladivostok and the surrounding waters. The drills included destroyers, corvettes, fleet oilers, submarine rescue ships, and—for the first time—submarines operating in tandem.

At the center of the patrol was Volkhov (B-603), a Russian Project 636.3 Improved Kilo-class diesel-electric submarine, accompanied on the surface by the Pacific Fleet’s corvette RFS Gromkiy (335). Across the horizon, the Chinese navy contributed CNS Urumqi (118), a modern destroyer, and CNS Xihu (841), a submarine rescue vessel. Lurking unseen was Great Wall 210, a Yuan-class submarine believed to have remained submerged for the entire operation.

Japan’s Maritime Self-Defense Force (JMSDF) reported detecting the Russian group twice, on August 14 and August 20, as it moved through the Tsushima Strait, one of Tokyo’s most critical maritime chokepoints. Significantly, Volkhov was spotted sailing on the surface, an unmistakable message: Russia wanted both Japan and the United States to know it was there.

Previous Sino-Russian naval exercises showcased destroyers, frigates, and aircraft. Submarines, however, have long remained absent from the partnership—until now.

Submarines are the most sensitive and secretive of naval assets. They demand rigorous water-space management (WSM) protocols to prevent collisions, acoustic deconfliction to avoid mutual interference, and shared recognition signals to distinguish friend from foe. That Moscow and Beijing were willing to attempt such coordination indicates an unprecedented level of trust.

“This is more than a training exercise—it is a baptism of trust,” one Asian naval strategist told this newspaper. “You do not risk operating submarines together unless you are building something much deeper: interoperability and shared deterrence.”

The symbolism is clear. Russia and China are not just exchanging signals of friendship—they are building procedures, testing interoperability, and laying foundations for future crisis operations.

The patrol’s implications extend far beyond the technical. Several strategic factors stand out:

• Operational Trust. Conducting a combined submerged patrol requires shared procedures and communications, something navies normally guard jealously.
• Geographic Messaging. By operating near Japan’s lifeline straits—Tsushima and La Pérouse—Moscow and Beijing reminded Tokyo that its front yard is contested.
• Calculated Risk. Entering heavily monitored waters shows confidence in the survivability of their platforms, and a willingness to accept heightened political risk.
• Shift in Signaling. Past drills were meant to be visible; submarine patrols emphasize ambiguity, uncertainty, and deniability—the very essence of undersea deterrence.
• Strategic Preparation. One day, Sino-Russian subs may cooperate to shield each other’s ballistic missile submarines (SSBNs) in so-called bastions during crises.
• Escalation Management. The patrol tested how the U.S. and Japan would respond to a combined undersea presence in real time.
• Forcing Allied Dilution. By drawing U.S. and Japanese anti-submarine resources north, China gains maneuver space around Taiwan and the South China Sea.
• Alliance Warning. Joint undersea patrols signal that Sino-Russian cooperation has matured from symbolism into functional coalition warfare.

“This was not a stunt,” warned a retired Japanese admiral. “It was a measured probe to see how quickly the JMSDF and U.S. Navy could detect, classify, and track a mixed Sino-Russian undersea presence.”

The patrol paired two complementary submarine classes.

Russia’s Improved Kilo-class boats are revered for their acoustic stealth, often dubbed “Black Holes” by NATO. Equipped with Kalibr-PL cruise missiles, they can strike targets up to 2,500 kilometers inland, transforming them from tactical ambushers into strategic strike assets.

China’s Yuan-class (Type 039A/B/C), represented here by Great Wall 210, employs air-independent propulsion (AIP), allowing it to remain submerged for weeks without surfacing. Armed with YJ-18 missiles—subsonic cruise missiles that accelerate to supersonic speed in their terminal phase—the Yuan poses a lethal threat to carrier strike groups.

Together, the pairing creates a layered strike profile: the Kilo optimized for stealth missile launches, the Yuan for endurance and area denial. For Japan and the United States, this complicates detection and stretches their anti-submarine warfare (ASW) resources thinner.

Few seas are as strategically sensitive as the Sea of Japan. Enclosed by Japan, Russia, and the Korean Peninsula, it is also a testing ground for Tokyo’s ASW doctrine.

Japan prides itself on one of the world’s most advanced submarine-hunting networks. It fields cutting-edge Soryu and Taigei-class submarines, sophisticated Kawasaki P-1 patrol aircraft, and seabed acoustic sensors spread across its chokepoints.

But combined Sino-Russian patrols push this system to its limits. Every encounter forces Japan to reveal how quickly it can mobilize, what detection techniques it uses, and how long it can sustain tracking. Each patrol provides Moscow and Beijing with a growing library of signals intelligence, eroding Japan’s tactical advantage.

For China in particular, the Sea of Japan offers invaluable exposure. Normally confined to the Yellow Sea and South China Sea, Chinese diesel-electric submarines rarely operate under such surveillance. Shielded by Russian cooperation, Beijing gains insights into allied tracking methods without risking unilateral exposure.

A subtle but significant aspect of the patrol was the participation of submarine rescue ships Igor Belousov (Russia) and Xihu (China).

Accidents involving submarines are politically devastating—recall Russia’s Kursk disaster in 2000. By integrating rescue vessels, Moscow and Beijing not only reduce risk but also display confidence in sustaining long-term undersea cooperation.

The message: these submarines will continue to push boundaries, and if disaster strikes, both navies have each other’s backs. This lowers operational caution and emboldens commanders to attempt riskier maneuvers in contested waters.

• Tokyo: Prepare for two nuclear powers simultaneously exerting undersea pressure near Japan’s chokepoints.

• Washington: The undersea monopoly that once belonged to the U.S. Navy is now being challenged by a coordinated bloc.

• Seoul: The Sea of Japan is no longer only about North Korean submarines—it is now contested by Chinese and Russian fleets.

• Taipei and Manila: If allied ASW resources are diverted north, the south—including Taiwan’s approaches and the South China Sea—becomes more permissive for the PLA Navy.

Operating together allows each navy to learn from the other’s strengths:

• Russia contributes Kalibr strike doctrine, wake-homing torpedoes, and experience in NATO-contested waters.

• China contributes AIP endurance, YJ-18 missile tactics, and cutting-edge combat systems.

Together, they can employ multi-axis submarine tactics, in which one sub acts as a decoy while the other maneuvers silently into position. For defenders, this splits ASW resources and risks oversaturation of detection networks.

The partnership also strengthens both navies’ ability to defend SSBN bastions: the Sea of Okhotsk for Russia, the Bohai Gulf and South China Sea for China. Joint operations could one day see Chinese and Russian submarines protecting each other’s nuclear deterrents.

Beyond torpedoes and missiles, the patrol underscores another vulnerability: undersea critical infrastructure.

Japan relies on seabed cables, energy pipelines, and surveillance arrays. Even without striking them, the presence of hostile submarines nearby forces Tokyo to invest heavily in monitoring and protection. This is grey-zone warfare at depth—applying pressure through presence rather than open attack.

The patrol will not go unanswered. Expect a wave of responses from the U.S., Japan, and South Korea:

• Increased ASW patrols using P-8A Poseidons and Kawasaki P-1 aircraft.

• Expanded seabed sensor networks and fixed arrays across chokepoints.

• Submarine shadowing, with U.S. Virginia-class and Japanese Taigei boats trailing Sino-Russian patrols.

• Trilateral drills focused on prosecuting multiple submarine contacts simultaneously.

• AI-driven acoustic processing, designed to detect ultra-quiet AIP and Kilo signatures in noisy littoral environments.

• South Korea accelerating deployment of KSS-III Batch II submarines, boosting both offensive reach and detection power.

Allied strategy will increasingly treat the Sea of Japan as a permanently contested battlespace, requiring 24/7 surveillance and layered ASW defense.

The first Sino-Russian joint submarine patrol is not a stunt—it is a strategic milestone.

A Kalibr-armed Kilo operating alongside an AIP-equipped Yuan creates a lethal synergy that challenges allied defenses. It signals that Moscow and Beijing are prepared to erode America’s last unchallenged maritime advantage: dominance beneath the waves.

Russia contributes decades of experience stalking NATO submarines in the Atlantic. China brings industrial scale, modern shipbuilding, and expanding undersea numbers. Together, they form a blend of quality and quantity that will demand constant allied attention.

If these patrols become routine, Northeast Asia could see the normalization of a two-power undersea presence, forcing permanent adjustments in allied budgets and force posture.