Jan 16 (Reuters) - Three of the six largest U.S. banks raised $23.25 billion in new bonds on Tuesday, kicking off an expected heavy week for new bank debt.

Wells Fargo tapped the bond market on Tuesday for $8 billion in fixed and floating-rate notes, in addition to senior unsecured fixed-to-floating rate paper.

Meanwhile, JPMorgan sold $8.5 billion in fixed and floating-rate notes, while Morgan Stanley issued $6.75 billion in fixed notes and floaters. Both banks' bond sales came in four tranches with different maturities of senior unsecured notes.

The deals follow a long list of Friday and Tuesday earnings releases from these and other global systemically important banks (GSIBs).

In addition to Wells Fargo, JPMorgan and Morgan Stanley, other banks that reported fourth-quarter earnings on Friday and Tuesday include Bank of America, BNY Mellon and Citigroup and Goldman Sachs.

January has historically been the biggest month for banks to issue bonds. According to data from Informa Global Markets, the last seven Januarys have seen an average $22.58 billion in issuance from the "Big Six" banks - JPMorgan, Citi, Bank of America, Wells Fargo, Goldman Sachs and Morgan Stanley.

The outlier was last January, which saw only $9 billion raised by the Big Six, according to the Informa data.

At least 11 investment-grade (IG) bond offerings, including Wells Fargo, JPMorgan and Morgan Stanley, are expected to price on Tuesday.

These include a $500 million three-year bond for cereal maker General Mills and a $400 million 10-year senior note for real estate investment trust Extra Space Storage .

Tuesday saw a total $30.38 billion in new IG corporate debt sales, according to Informa Global Markets data, bringing January's tally to $133.67 billion.

The heavy supply comes despite the holiday-shortened week. For the week of Martin Luther King Jr Day, the average investment-grade bond supply dating back to 2016 has been $24.6 billion, according to Daniel Krieter, director of fixed income strategy at BMO Capital Markets.

Banks are expected to be particularly active in debt issuance this month, as they look to get ahead of new regulatory requirements to maintain higher capital reserves.

"That makes a projection of $35bn from U.S. GSIBs in January reasonable in our estimation, with the majority of that supply expected to come this week," Krieter wrote in a Tuesday note. (Reporting by Matt Tracy, Editing by Nick Zieminski)