I discovered in 2018 that I could have used HSA for my 2017 COBRA premiums. I want to use the HSA to reimburse myself in 2018 for the 2017 COBRA premiums

[tunkiemo]

Will I have 2018 tax implications for 2017 reimbursement

[xmasbaby0]

Did you itemize medical expenses for your 2017 return and enter the COBRA premiums?

[Opus 17]

First, you can't reimburse yourself for any expense unless the expense was incurred after the date of the opening of the HSA. 

Then, if you took an itemized deduction for medical expenses last year and included the COBRA premiums, you will have to make an adjustment on this year's tax return (you report a taxable recovery, or reimbursement of  a previous deduction.)  The amount of adjustment may not be obvious without a calculation we can tell you if you have to do it.

And your contributions IN to the HSA for 2018 are still limited by either the personal or the family limit (plus the over-55 catchup provision if that applies to you).

Taking those factors into account, it is perfectly acceptable to deposit money in the account to qualify for the tax deduction then immediately withdraw it to reimburse yourself for a past medical bill.