Oleksiy Kasyanov or Oleksii Serhiiovych Kasianov (Ukrainian: Олексій Сергійович Касьянов; born 26 August 1985 in Stakhanov (now Kadiivka, Ukraine) is a Ukrainian decathlete.
His personal best score is 8479 points, achieved at the 2009 World Championships in Berlin, Germany. He won the 2010 TNT-Fortuna Meeting in Kladno with a total of 8381 points, leading from the first day onwards with the help of a 14.24 seconds personal best in the 110 metres hurdles.
Since 2014, he is married to Ukrainian heptathlete Hanna Melnychenko.