The Opel Senator is a full-size executive car (E-segment) produced by the German automaker Opel, two generations of which were sold in Europe from 1978 until 1993. A saloon, its first incarnation was also available with a fastback coupé body as the Opel Monza and Vauxhall Royale Coupé. The Senator was, for its entire existence, the flagship saloon model for both Opel and Vauxhall.

Through the international divisions of General Motors, it was also known in various markets as the Chevrolet Senator, Daewoo Imperial (in South Korea), Vauxhall Royale (until 1983) and Vauxhall Senator (which took the place of the Royale on Vauxhall models when the Opel brand was phased out from 1983). It was also sold as the Opel Kikinda in Yugoslavia, where it was produced under licence by IDA-Opel in Kikinda, Serbia, after which it was named.

The original Senator was a de facto replacement for Opel's KAD cars (the Opel Kapitän, Admiral and Diplomat), which competed in the F-segment (full-size luxury) in which the KAD cars had sold poorly. Sister company Vauxhall had already abandoned the segment with the demise of its Cresta/Viscount models some years earlier, leaving the Ventora model (a luxury derivative of the FE Victor/VX4) as its flagship offering but this was axed in 1976 with no direct replacement.

The Senator shared its platform with the smaller Opel Rekord, the latter being lengthened to make the Senator. The second generation of that car, from 1987, shared its base with the Rekord's Opel Omega successor, which was again lengthened to produce the Senator.

The Senator A was last part of a joint model programme executed by GM in the 1970s to develop a common series of vehicle platforms for both its European brands (and also for the Holden brand in Australia). The first two product families of this strategy – the T-Car (Kadett C/Chevette) and the U-Car (Ascona B/Manta B/Cavalier) had already been released. The V-Car (or V78) platform would simultaneously provide the next-generation Opel Rekord, a replacement for the Vauxhall FE Victor, and a 'stretch' version to replace the unsuccessful 'KAD' cars, and act as a flagship for both Opel and Vauxhall.

The Senator therefore emerged as a long wheelbase version of the Opel Rekord E, complemented by a three-door fastback coupé version on the same platform called the Opel Monza, which was intended as a successor for the Opel Commodore coupé.

The Senator A and Monza were initially sold in the United Kingdom as the Vauxhall Royale (and Vauxhall Royale Coupé). Unlike other members of the joint Opel/Vauxhall model programme of the period, the Royale was simply a badge engineered version of the Senator with only detail differences from its Opel sister.

Following the merger of the UK Opel and Vauxhall dealer networks in 1982, the Opel marque was repositioned as a performance-luxury brand, and the Vauxhall Royale models were dropped in favour of the Opel Senator/Monza, coinciding with the "A2" mid-cycle facelift. This policy was reversed in late 1984, with the Senator reverting to Vauxhall branding for the 1985 model year, but the Monza remained on sale as an Opel until its discontinuation at the end of 1987.