If I add additional funds to my 2019 401K in 2020 will that help to decrease my taxable income?

[mlewis01]

If I add additional funds to my 401K for 2019 in 2020, will that decrease my taxable income for 2019?

[SusanY1]

If you are contributing to a Solo(k) then yes, you can still make 2019 contributions that will lower your 2019 taxable income as long as the plan was set up prior to the end of the year. 

You have until the due date of your company's return, plus extensions, to fund the plan and reduce your income for the year. 

If you are an S-corporation or Partnership your company's return was due March 15th unless extended.  (However, S-corp wages would need to have been reduced on W-2 prior to year-end.)

If you file your business taxes on a Schedule C then you have until July 15th, 2020 to make contributions unless you file an extension.  If you extend your tax return deadline, you will have until October 15, 2020 to make these contributions (for Schedule C).  

Employees contributing to a plan via payroll deductions that are reported on their W-2s must make these contributions (or determine the amount) within the calendar year.