The 6600 series was a family of mechanical watch movements from Seiko in the 1960's.

Produced from 1961, the 6600 family was the basis of many later Seiko movements, including the 6100 family. It ran at 18,000 A/h and used an automatic winding mechanism, Seiko's Magic Lever. The family was available in varying jewel counts 17, 19, or 21.

The first member of the lineup was the 1961 6601A. Available with 17, 19, or 21 jewels and measuring 5.45 mm high, this basic movement nevertheless had a sophisticated automatic winding system. It was replaced by the 6601B later with unknown modifications. The 6606B and 6619A were launched in 1965 with a non-quickset day and date wheel and 21 jewels. It was not until the 1968 6602B that Seiko reached a 45 hour power reserve, and this movement was 1 mm thinner.

• Hand-wind - Officially 23.6 mm in diameter
  • 436/4361/66A (1960) - 23.6 or 24.1 mm, hand-wind
  • 66B (1960) - 23.6 or 24.1 mm, hand-wind
  • 957/6602B - 27.6 or 28.1 mm, hand-wind, dater Cal 66
  • 6618A (1966) - 24.1 mm, hand-wind
  • 6640A (1961) - 24.1 mm, hand-wind
  • 6642A - hand-wind
  • 6659A (1964) - 24.1 mm, hand-wind
  • 6660A - 24.1 mm, hand-wind
• Automatic
  • 6601A (1961) - 27.6 or 28.1 mm, automatic
  • 6601B - 27.6 or 28.1 mm, automatic
  • 410?/6606B (1963) - 27.6 or 28.1 mm, automatic, day/date
  • 6619A (1964) - 27.6 or 28.1 mm, automatic
  • 245/2451
  • 790
• 6605 (1962)
• 660