The IRS treats bitcoin and other cryptocurrency transactions as property and not as currency. As such, you must figure gain on the transaction in the same way as with any other type of property. The IRS posted guidance (https://www.irs.gov/pub/irs-drop/n-14-21.pdf) on this matter several years ago, but may revisit the topic, so stay alert.
Property subject to capital gain or loss treatment. Almost all assets you own are considered capital assets except for certain business assets or works you created.