A functional unit for supporting multithreading, a processor including the same, and an operating method of the processor are provided. The functional unit for supporting multithreading includes a plurality of input ports configured to receive opcodes and operands for a plurality of threads, wherein each of the plurality of input ports is configured to receive an opcode and an operand for a different thread, a plurality of operators configured to perform operations using the received operands, an operator selector configured to select, based on each opcode, an operator from among the plurality of operators to perform a specific operation using an operand from among the received operands, and a plurality of output ports configured to output operation results of operations for each thread.