There are differing opinions on this, but my opinion is yes. Under IRC 404(a)(6), the contribution deemed to have been made on the last day of the prior taxable year if it is made "on account of such taxable year." Clearly it was made on account of the prior plan year, but what does it mean to be made on account of the prior taxable year? The IRS doesn't elaborate, but the only thing that makes sense to me is that the employer deducts it on that year's tax return. Since they didn't do that, I would argue that it defaults to being deductible in the year contributed (assuming it meets all the other requirements to be deductible, of course).