The Delta class, (Russian: Дельта) Soviet designations Project 667B Murena, Project 667BD Murena-M, Project 667BDR Kalmar, Project 667BDRM Delfin, (NATO reporting names Delta I, Delta II, Delta III, Delta IV respectively) are a family of nuclear-powered ballistic missile submarines, designed and built in the Soviet Union, which formed the backbone of the Soviet and Russian strategic submarine fleet since their introduction in 1973. They carry nuclear ballistic missiles of the R-29 Vysota family, with the Delta I, Delta II, Delta III and Delta IV classes carrying the R-29/SS-N-8 'Sawfly', R-29D/SS-N-8 'Sawfly', R-29R/SS-N-18 'Stingray' and R-29RM/SS-N-23 'Skiff' (and later on improved versions) respectively.

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The Soviets viewed the Deltas as an iterative improvement of the Yankee-class submarines, which carried R-27 Zyb missiles with a range of 2,500–3,000 km (1,553–1,864 mi). The R-29s gave the Deltas much needed standoff distance; with a range of 7,700 km (4,785 mi) the Deltas were able to perform their deterrence patrols within relative safety of the Arctic Ocean, while the Yankee-class had to patrol off the US coastline to do so. The Deltas were supplemented by the largest submarines ever built, the Typhoon-class submarines, which served as guarantors of the Soviet second strike capability. The earlier Delta boats remained in service until the 1990s, when the Soviet Union ceased to exist and many classes of submarines were decommissioned due to the severe budget cuts that resulted. A few Delta-IIIs and all of the Delta-IVs were retained by the nascent Russian Navy.

34 boats were built and commissioned during 1972–1990; approximately five or six remain active in 2023. A handful were converted into special-purpose submarines operated by GUGI.

In the 1960s the Soviet Navy wanted new submarine-launched nuclear missiles that could threaten targets in North America without their launch platforms needing to pass the SOSUS sensors in the GIUK gap to be within range.^[1] The resulting project was an iterative improvement of the Project 667A Navaga; The 667B could accommodate larger, more capable missiles within its enlarged dorsal hump.

The Project 667B Murena/Delta I submarines could deploy on combat patrols in the marginal sea ice of the Soviet Arctic coastal zone, and often patrolled in the Norwegian and Barents Seas. Consequently, unlike their predecessors, they no longer needed to pass through Western SOSUS sonar barriers to come within range of their targets. To improve the accuracy of the missiles, the Delta I-class submarines carry the Tobol-B navigation system and the Cyclone-B satellite navigation system.

After construction was authorized in 1965, the first Delta I, K-279, was commissioned into the Soviet Northern Fleet on 22 December 1972. A total of 18 submarines of this class were built, and all served the Soviet Navy.

In 1991, nine Delta I-class submarines were still in active service. Their decommissioning began in 1994, with removal of the missile compartments scheduled by 1997. All submarines of this class were taken out of service by 2004 and were scrapped by 2005. The last active submarine of this project was the K-447 Kislovodsk, which was in service for more than 30 years and was decommissioned on March 5, 2004.^[2]

The 667BD Murena-M/Delta II submarines were designed to remedy the shortcomings of the Delta I submarine. The planform was largely the same, but the submarine was lengthened in the fourth and fifth compartments by 16 meters (52 ft) to allow the installation of four more missile tubes. The Delta-IIs also received additional quieting measures such as mounting the steam turbines on shock absorbers, having all pipes and hydraulics separated from the hull through rubber insulation, and a special hydroacoustic coating being applied to the hull.^{[citation needed]}

Only four submarines of this class were built, apparently in favor of building the following class, the Delta III, and all Delta IIs were out of service by 1996.

The 667BDR Kalmar/Delta III-class submarine is a further development of Project 667, maintaining the double-hulled design with a thin, low magnetic steel outer hull wrapped around a thicker inner pressure hull. Development began in 1972 at the Rubin Central Design Bureau for Marine Engineering.^{[citation needed]} The submarine was the first that could launch any number of missiles in a single salvo, as well as the first submarine capable of carrying ballistic missiles with multiple independently targetable reentry vehicles. The submarine carried 16 of the R-29R missiles each carrying 3 to 7 MIRVs, with a range of 6,500 to 8,000 km (4,000 to 5,000 mi), depending on the number of re-entry vehicles.

As of 2023 two Delta-IIIs remain; K-44 Ryazan, and the heavily modified BS-136 Orenburg.

Seven Project 667BDRM Delfin/Delta IV-class submarines were built, which were yet another iterative improvement of the Delta-class. All were retained and operated by the Russian Navy. The submarines, based at the Sayda Guba Naval Base, operate in the Northern Fleet. The Severodvinsk Shipyard built these vessels between 1981 and 1992. The last vessel completed was K-407 Novomoskovsk.

• List of Soviet and Russian submarine classes
• Future of the Russian Navy
• Submarine-launched ballistic missile