The invention discloses a telescopic and length-adjustable rear center armrest which comprises an armrest body, an extended armrest body and a locking mechanism. A backrest of a seat body is provided with a mounting groove, and the bottom of the armrest body is hinged to the bottom of the mounting groove; the armrest body is provided with a groove, and the extended armrest body is installed in the groove; guiding grooves are formed in two side arms of the groove, and sliding blocks capable of sliding inside the guiding grooves are arranged on the two sides of the extended armrest body; the locking mechanism can lock the position of the extended armrest, the extended armrest body can stretch out and draw back, the armrest body can be stored in the mounting groove of the backrest, space inside a vehicle is not occupied, and the comfort requirement can be met.