PROBLEM TO BE SOLVED: To provide a signal processing device, an underwater detection device, a signal processing method, and a program, capable of appropriately displaying echoes from a target object without requiring cumbersome work.SOLUTION: A signal processing device allows an underwater detection device transmit an acoustic wave and processes each of a plurality of received data sets constituting a returned reception signal, and includes; a gain computation unit which computes a gain corresponding to elapsed time from reference time to reception time of the received data; a lower limit gain computation unit 41 which extracts a predetermined received data set from the plurality of received data sets and computes a lower limit gain based on the extracted received data set; a comparator unit which compares the gain and the lower limit gain and outputs the gain when the gain is greater than the lower limit gain and outputs the lower limit gain when the gain is the lower limit gain or lower; and a correction u
