I bought a 3 month treasury bill on the secondary market and sold it prior to maturity one month later.
Treasury Bills are bought at discounts and there is no coupon.
Is the gain on the sale reported as interest income or a short term capital gain?
you could have both interest income and capital gain/loss. the discount is prorated for the period you held the bill and would be added to your purchase price to come up with your tax basis for the purposes of the sale. this is an opinion because I could find nothing specific. you could check with the financial institution through which you sold it.