The Delta IV-class submarines

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 series 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 (NATO reporting name: SS-N-8 'Sawfly'), R-29D (SS-N-8 'Sawfly'), R-29R (SS-N-18 'Stingray') and R-29RM (SS-N-23 'Skiff') respectively. The Delta I class carried 12 missiles, while the Delta II class which are lengthened versions of the Delta I class carry 16 missiles. The Delta III and Delta IV classes carry 16 missiles with multiple warheads and have improved electronics and noise reduction.

The R-27 Zyb missile carried by the Yankee-class submarines of the late 1960s had a range of 2,500–3,000 km (1,553–1,864 mi), so the earlier submarines were forced to patrol close to the North American coast, whereas the Deltas could launch the over 7,700 km (4,785 mi)-range R-29s from the relative safety of the Arctic Ocean. In turn the Deltas were superseded by the larger Typhoon-class submarines. The early Deltas remained in service until the 1990s with treaties such as START I. High running costs and the retirement of the Typhoons' R-39 missiles meant that some Delta III-class submarines were reactivated in the 2000s (decade) to replace the Typhoons.

In December 2010, Pavel Podvig and russianforces.org estimated the strength of the Russian strategic submarine fleet at one Typhoon-class submarine (used to test the RSM-56 Bulava missile), four Delta III, six Delta IV class, and one Borei class strategic missile submarines. They will ultimately be replaced by the new Borei class, also referred to it as the Dolgorukiy class.