The twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a key mite pest affecting blackberry production worldwide. Tetranychus urticae feeds on the underside of leaves, extracts chlorophyll, and reduces crop yield. Amblyseius (Neoseiulus) californicus (McGregor) (Acari: Phytoseiidae) has been identified as a potential predator of T. urticae. We conducted a greenhouse and a field experiment to evaluate the potential of N. californicus as a biological control agent for T. urticae on 'Arapaho,' 'Navaho,' and 'Quachita' blackberry varieties. Research on N. californicus-based biological control has not been conducted previously in blackberries because the crop matures during the summer when temperatures are high, and there are concerns whether N. californicus can control T. urticae populations during these high temperatures. The experimental design was a completely randomized block, and treatments included the following: (1) abamectin, (2) N. californicus, and (3) untreated blackberry plants. Abamectin was effective, but mite populations were cyclic and additional applications were needed. The study demonstrated that N. californicus provided the most effective and sustained control for T. urticae on blackberry plants under hot and humid conditions. A good assessment of local phytoseiids and other predators is needed before releasing N. californicus into blackberry plantings.