Thomas P. Barnett, also known professionally as Tom Barnett and Tom P. Barnett, was an American architect and painter from St. Louis, Missouri. Barnett was nationally recognized for both his work in architecture and in painting.

Barnett trained under his father, St. Louis architect George I. Barnett, who was known for designing public landmarks such as the renovation of the Old Courthouse, the Missouri Governor's Mansion, and the structures of the Missouri Botanical Garden. In painting, the younger Barnett trained at the St. Louis School of Fine Arts , with a fair list of awards and exhibitions.

After graduating Saint Louis University in 1886, Tom Barnett joined with his brother and brother-in-law, George Dennis Barnett and John Ignatius Haynes, to form the architectural firm Barnett, Haynes & Barnett. The firm continued the traditional motifs of the elder George Barnett. The combined legacy of two generations of Barnett designs were largely responsible for Classicism being the dominant architectural influence in St. Louis.

In 1904, Barnett served on the Commission of Architects for the Louisiana Purchase Exposition (better known as the St. Louis World's Fair) and personally designed the Palace of Liberal Arts for which he earned the fair's Gold Medal for Architecture. The following year, he would win the Bronze Medal for Architecture at the Lewis and Clark Centennial Exposition in Portland, Oregon.

Other projects designed by Barnett included commercial buildings, residential (including private places), and a significant number of religious structures. Surviving examples include the Cathedral Basilica of St. Louis, the Adolphus Hotel in Dallas, Texas, and the Saint Clement Catholic Church in Chicago.