I am a single, 58 year old man. My 21 year old son and I are covered under my high deductible health plan. Can I contribute $8100 (pre-tax dollars) to my HSA in 2021?

[Critter-3]

Yes you can IF  the insurance is HSA qualified  ... just being a HDHP is not enough.  Ask the insurance company if your plan is HSA qualified.

[rbj-1]

Yes, the health plan is HSA qualified. 

[NCperson]

and the 2021 amount is $8200 

[Opus 17]

Yes ($8200 for 2021).  Not only that, your son can open an HSA in his own name and contribute $7200, as long as he does not have some kind of disqualifying secondary coverage like Medicaid or a PPO or FSA from his employer or from his other parent.

There is a rule that spouses can only contribute the same overall family maximum ($7200), split any way they want.  But Congress seems to have overlooked the situation of adult children.  If your son is "covered" by a qualifying family plan (and he is), then he can contribute up to the family limit into a separate account in his own name, even though he is not the owner of the insurance policy.  He might not save much in taxes, depending on his other income, but the savings would grow tax-free for future medical expenses or retirement.  An HSA makes an excellent retirement account and he can contribute even if he is not allowed to contribute to an IRA due to a lack of earned income. 

[Opus 17]

Also, don't forget that if you are covered by a qualifying plan on December 1, 2021, you and your son can each make full year contributions for tax year 2020 ($8100 and $7100) under the "last month rule".  You can make 2020 contributions up until April 15, 2021, as long as you tell the HSA bank that the contribution is designated for 2020 when you make the deposit.