Bitcoin Jumps 20% to a Six-Week High

The price of bitcoin jumped to a six-week high on Monday, with some investors attributing the rally to short positions being liquidated and speculation that Amazon . com Inc. may be venturing into digital currencies.

Bitcoin soared to $39,112.05, up more than 20% from its 5 p.m. EDT level Friday, according to CoinDesk, reaching its highest level since mid-June. Ether, its rival currency, jumped more than 16%.

Recent bullish comments from high-profile proponents of cryptocurrencies have helped buttress the price gains. Last week, Tesla Inc. Chief Executive Elon Musk said he and his rocket company, SpaceX, hold bitcoin, despite concerns about its environmental impact. Mr. Musk also said Tesla would likely accept the cryptocurrency as payment again if the process of generating it, known as mining, becomes less reliant on fossil fuels.

“There’s been a lack of good news in the cryptocurrency market in the last two months,” said Bobby Lee, founder and CEO of Ballet, a cryptocurrency hardware wallet. “Now they’re starting to trickle out, so investors and speculators are using this opportunity to build up their positions and buy back bitcoin, causing the price to rise pretty dramatically,” added Mr. Lee, who splits his time between Shanghai and Las Vegas.

Bitcoin is still about 40% below its high of nearly $65,000 in mid-April. The following month, China’s renewed efforts to crack down on bitcoin mining and trading contributed to sharp price declines.