Cook told the paper in an interview that Apple was "surprised" by the 8 percent decline in its shares on January 28, the day after it reported quarterly results.
Lower-than-expected holiday iPhone sales and a weak revenue forecast by Apple renewed fears about lower Chinese demand and a tepid global market.
With the latest round of purchases, Apple had bought back more than $40 billion of its shares over the past 12 months, which according to Cook was a record for any company over a similar span, the Journal said.
"It means that we are betting on Apple. It means that we are really confident on what we are doing and what we plan to do. We are not just saying that. We are showing that with our actions," Cook told the Journal.
The share buyback was part of Apple's previously disclosed plan to repurchase $60 billion of its own shares.
Apple had returned $7.7 billion to shareholders through dividends and buy backs in the December quarter, bringing the total under its capital return program to $43 billion, excluding latest purchases.
Cook told the paper that Apple plans to disclose "updates" to its buyback program in March or April.
Apple could not immediately be reached for comment by Reuters outside of regular U.S. business hours.