IRS Material: SE Tax Deduction
Self-employment tax. You can deduct one-half of your self-employment tax as a business expense in figuring your adjusted gross income. This is an income tax adjustment only. It does not affect your net earnings from self-employment or your self-employment tax.
To deduct the tax, enter on Form 1040, line 26, the amount shown on the "Deduction for one-half of self-employment tax" line of the Schedule SE (Form 1040).
For more information on self-employment tax, see Publication 533.