Wat Tha Sai, or the Monastery of the Sand Landing, was located off the city island in the northwestern area of Ayutthaya, in Wat Tum Sub-district. The monastery stood on the south bank in an oxbow of the old Lopburi River, just like Wat Chang Yai, Wat Chang Noi (pavilion) and Wat Khae (defunct). It was situated west of Wat Chang Yai, east of Road No 347. Wat Long Thong (ruin) and Wat Chumphon were south.

In situ is a large brick mound with remains of a wall and a chedi. Some brickwork can be seen being enveloped in a tree. The whole site is grown over by vegetation, while some parts are inundated.

Its historical background and period of construction are unknown.

The site is shown on a Monthon Krung Kao map of 1916 CE, on a military map Changwat Phra Nakhon Si Ayutthaya - L708 Edition 1 - RTSD Thailand 1:50000 (1960) Sheet 5154 III within ND 47-8 L509, and is also found on the 1993 CE and 2007 CE Fine Arts Department maps.

The ruins of Wat Tha Sai are in geographical coordinates: 14° 24' 4.06" N, 100° 31' 50.98" E.