Leonid Isaakovich Mandelstam or Mandelshtam (Belarusian: ?????? ????????? ???????????, Russian: ?????? ????????? ???????????, IPA: [l???'n?id is?'รคk?v?it? m?nd??l?'?tam] (listen); 4 May 1879 โ 27 November 1944) was a Soviet physicist of Belarusian-Jewish background.