Development of spider mite management technology other than chemical control is desired because of the serious development of acaricide resistance worldwide. Recent studies have evidenced the lethal effects of ultraviolet-B (UVB) radiation on spider mites. To develop the technology on how to use UVB irradiation for spider mite control, we tested whether UVB lamp–light reflection sheet (LRS) combinations suppressed the population size of Tetranychus urticae Koch on strawberry in a greenhouse from December to May (2012–2013, 2013–2014) in Japan. We designed four combinations of UVB lamps and LRSs: 1) neither UVB lamps nor LRSs (UV−LRS−); 2) a UVB lamp without an LRS (UV+LRS−; 2012–2013 only); 3) a UVB lamp and a mulch-type LRS (UV+LRSm); and 4) a UVB lamp and a wing-type LRS (UV+LRSw). The number of adult females peaked at 438.0 and 222.0 per plant in UV−LRS− of 2012–2013 and 2013–2014, respectively, and peaked at 191.6 females in UV+LRS− of 2012–2013. In contrast, the peak abundance was 20.9–98.0 females in UV+LRSm, and fewer than 15 females were noted in UV+LRSw over either experimental period. UVB irradiance on lower leaf surfaces was higher in UV+LRSm and UV+LRSw than UV−LRS− and UV+LRS−, and the mite densities were significantly correlated with UVB irradiance on lower leaf surfaces. Consequently, we conclude that the combinations of UVB lamp–LRS have an excellent capacity to control T. urticae on greenhouse strawberry, and that the LRS was an essential component in this technological approach.