The central bank held a total of 690 trillion yen ($6.6 trillion) in assets as of end-September, up 21% from a year ago and nearly 1.3 times the size of Japan's economy.
The increase was largely due to the introduction in March through May of a range of lending schemes aimed at easing corporate funding strains caused by the pandemic.
Loans extended during April-September more than doubled from year-before levels at 105 trillion yen, the data showed.
The BOJ's holdings of exchange-traded funds (ETF) also rose to a record 34 trillion yen, as it accelerated purchases when markets took a hit from the spread of COVID-19.
The ETF holdings booked unrealised profits of 5.8 trillion yen, the second largest on record, thanks to a rebound in the stock market later in the year, the data showed.
The BOJ eased policy twice this year, mainly by ramping up asset purchases and creating a scheme aimed at funneling funds to cash-strapped firms hit by COVID-19.
(Reporting by Leika Kihara; Editing by Toby Chopra)