Only during the very brief period between the resignation of Stettitnius as Secretary of State and the appointment of Byrnes. The OP asked about the entire period of Truman's presidency before his election in 1948, not about one week in late June-early July 1945!
For most of the period of Truman's first term, the Secretary of State--first Stettinius, then Byrnes, then Marshall--would be the successor. After the passage of the Presidential Succession Act of 1947, it would be Speaker of the House Joe Martin.