The clubs are believed to have agreed a fee in excess of £8m and Caulker has agreed terms on what is reported to be a five-year deal.
Caulker, 21, started 18 Premier League games for Tottenham last season and spent the previous season on loan with Cardiff's rivals Swansea City, playing a further 26 top-flight games.
He made his senior England debut last November and scored in the 4-2 friendly defeat by Sweden.
Caulker would become Cardiff's fourth summer signing and, if the fee proves to be accurate, their new record signing, surpassing the £7.5m they paid for FC Copenhagen striker Andreas Cornelius on July 1.
Derby County defender John Brayford and Brentford goalkeeper Simon Moore have also joined the club this summer and boss Malky Mackay is keen on bringing in Toulouse midfielder Etienne Capoue as well.