Isobutene ((CH_3_)_2_C=CH_2_) is one of the four isomers of butene (C_4_H_8_). After the detection of propene (CH_3_CH=CH_2_) toward TMC-1, and also in the warmer environment of the solar-type protostellar system IRAS 16293-2422, one of the next alkenes, isobutene, is thus a promising candidate to be searched for in space. We aim to extend the limited line lists of the main isotopologue of isobutene from the microwave to the millimeter region to obtain a highly precise set of rest frequencies and to facilitate its detection in the interstellar medium.