An internal voltage trimming device and a semiconductor integrated circuit including the same are provided. The internal voltage trimming device includes a voltage dividing circuit configured to generate a feedback voltage based on a resistance of the voltage dividing circuit and a target voltage that is received in a trimming mode, and a comparator configured to compare a reference voltage and the feedback voltage to generate a comparison signal. The internal voltage trimming device further includes a direct current to direct current (DC-DC) converter configured to generate an internal voltage based on an input voltage and the comparison signal, and be disabled in the trimming mode, and an automatic trimming circuit configured to generate, in the trimming mode, a trimming signal based on the comparison signal. The voltage dividing circuit is further configured to adjust the resistance based on the trimming signal.