The price of bitcoin rose above $9,000, extending a rebound that has made cryptocurrencies far outperform traditional asset classes this year.

Bitcoin on Monday reached $9,396.33, a 13-month high, according to CoinDesk, a research site. While stocks, bonds, gold and oil are all up this year, bitcoin’s rally trounces them all. Prices for bitcoin and rival digital currencies collapsed last year.

Investors...