A telescoping countertop support bracket assembly includes a channel member that defines a plurality of through holes through a lateral wall of the channel member. Each of the through holes is configured to receive a fastener; the channel member defines a support bar receiving portion and an accessory bar receiving portion. A support bar is received by the channel member, and it is telescopically adjustable within the channel member. A countertop contact surface is supported by the support bar, and the fasteners secure the support bar within the channel member at an adjustable support length.