Lys05, or Lys01 trihydrochloride, is a potent, water soluble lysosomal autophagy inhibitor. Lys05 is a previously undescribed dimeric chloroquine which more potently accumulates in the lysosome and blocks autophagy compared with HCQ. Lys05 produced more potent antitumor activity as a single agent both in vitro and in vivo in multiple human cancer cell lines and xenograft models compared with HCQ. Lys05 is therefore a new lysosomal autophagy inhibitor that has potential to be developed further into a drug for cancer and other medical applications.