Yes, land in Washington, D.C. can be privately owned. While the federal government owns a significant portion (including the National Mall, federal buildings, parks, and military installations), a substantial amount of land is held by private individuals, businesses, and organizations.
Property rights in D.C. function similarly to those in states, with deeds, titles, zoning regulations, and property taxes administered by the District government. The unique aspect is the federal presence, but private ownership is fully legal and common.