In 2012-2014 we obtained 3641 CCD frames of the fields of the RR Lyrae (AB subtype, P = 0.583 days) variable DU Mon with BV I-c filters using the 76-cm telescope of the South African Astronomical Observatory (SAAO) and the 1-m telescopes of the Las Cumbres Observatory Global Telescope Network (LCOGT). Our observations confirmed the presence of the Blazhko effect that we suspected previously and allowed its period to be determined, P-Bl - 60.(d)52 +/- 0.(d)03. Using all of the available observations, we constructed an O-C diagram spanning a time interval of 86 years that revealed at least one abrupt change in the pulsation period (a decrease by 15.26 s).