Cal. 35 SQ (also called Cal. 35A) was the first quartz movement from Seiko and the first production quartz movement sold to the public. It was produced from 1967, offered for sale on December 25, 1969, and continued in production for a year or so after.

Seiko was quietly racing to develop a quartz watch movement in the 1960s, and delivered their first Cal. 35 movement to the Neuchatel Observatory in late 1967, just a few months after the CEH delivered their "Beta" quartz movement prototypes. Seiko's movements were not thermocompensated so they performed poorer than the Swiss movements, but they incorporated modern design touches including a stepper motor rather than the vibration motor used by the Beta movements. Cal. 35 was a product of Suwa Seikosha and had an 8,192 Hz quartz crystal. The integrated circuit used to count quartz vibrations was produced by Intersil in the USA.

Seiko offered Cal. 35 SQ for sale in the Seiko Astron watch on Christmas Day, 1969. This marked the first quartz watch available for sale, though Longines had announced their Ultra-Quartz five months earlier. Seiko claimed to have sold about 100 examples in the first month, and delivered perhaps 1,500-1,800 examples total in a one-year period. The first deliveries took place in January 1970, well before any Swiss quartz watch.

The initial Astron watches used Cal. 35 SQ, but a date complication was soon added and the crystal was upgraded to 16,384 MHz. Cal. 35 SQC (with date) was somewhat more common later in the year. Both movements are called Cal. 35A officially.

Seiko introduced Cal. 3600 in 1970 as well, but neither was produced in large volumes. Suwa's next quartz movement, Cal. 38 SQW was more production-ready and went on sale in late 1971. The Cal. 38 series marked the first true production quartz watch from Seiko.

• Suwa Seikosha 35 Quartz Stream