In this letter, an adaptive space-time shift keying (ASTSK) system is proposed, where the transmission modes of each transmit antenna are dynamically changed based on varying channel state information (CSI) fed back to the transmitter. ASTSK can achieve better bit error rate (BER) performance than that of space-time shift keying (STSK). To further utilize CSI, a candidate selection of the number of transmit antennas is additionally introduced before utilizing ASTSK, termed concatenated adaptive space-time shift keying (CA-STSK). Furthermore, Chernoff bounds of the pairwise error probability of ASTSK and CA-STSK are derived. The simulation results illustrate that both ASTSK and CA-STSK have significant improvements in BER performance compared to conventional STSK. Additionally, compared with ASTSK, CA-STSK demonstrates superior BER performance.