Assuming the per diem was a reimbursement for work related travel, including meals. you may have a deduction for travel expenses that can be used to offset the income through itemized deductions. Use Deductions & Credits for all itemized deductions.
In TurboTax for job related expenses specifically do the following:
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No. Starting in 2018 W-2 job related expenses are no longer deductible. So if the per diem was included in box 1 of your W-2, it's taxable income to you.
You may wish to talk with your employer about an "accountable" reimbursement plan. This is a plan where you provide the employer receipts for your qualified W-2 job related expenses and they reimburse you. Then the amount reimbursed is not included on your W-2 and you therefore don't get taxed on it.