Both forms (Thomas' and Thomas's) are correct singular possessive nouns.
There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the word: Thomas'
Add an apostrophe s ('s) after the existing s at the end of the word: Thomas's
Examples:
Dr. Thomas' office is on the second floor.
Dr. Thomas's office is on the second floor.
Use the form that best suits the way you say the word, or if you are a student, the form your teacher prefers.
Either is correct. According to the Oxford English Dictionary, it is perfectly acceptable to place an apostrophe and s after an already existing s in singular nouns.
Chantal Thomass was born on September 4, 1947.
Michel Thomass was born on August 21, 1912, in Moscow.
Eugen Thomass was born on December 24, 1927, in Munich, Germany.
Rosalie Thomass was born on August 14, 1987, in Munich, Germany.
Eugen Thomass died on January 21, 2009, in Winterthur, Zurich, Switzerland.
composers
Thomas and Friends Steamies vs- Diesel and Other Thomas Adventures - 2004 was released on: USA: 18 May 2004
Divorce Court - 1999 Wilson vs- Thomas was released on: USA: 25 October 2011
Cristina's Court - 2006 Thomas vs. Prietto was released on: USA: 2 January 2012
Crime and Punishment - 2002 People vs- Tianna Thomas and People vs- Charles Mambane 3-6 was released on: USA: 2004
The cast of Bruce Lee vs. Peter Thomas - 2013 includes: Brandon Bentley as Narrator
American Gladiators - 1989 Gold Medal Challenge of Champions III Gaylord vs- Ctvrtlik vs- Thomas vs- Street 7-20 was released on: USA: 1995