Yes. All forms of marijuana are illegal under federal law. However, the District of Columbia legalized medical marijuana in 2010, and over a hundred retail outlets have been issued licenses. This means that even if you're in possession of small amounts of cannabis, you will not be at risk of being arrested by the police if you're in one of these locations. However, you must have registration documents with you to prove that you are a legitimate patient.
DC is having a unique legislation for marijuana. It was the first in the nation to legalize marijuana. However, you are still considered to have broken the law if you smoke in public places. You can be fined by $100, but the most that you can be charged is $500 if you are smoking in a private place. At the same time, you can buy and use weed as long as you are above 21. You need to be able to prove your age to the police if ever you are caught. This is unique legislation so you need to know what exactly you can and cannot do.
There are some federal laws that apply to the District of Columbia. The District of Columbia is a federal district and we have no voting representatives in Congress. The U.S. Constitution, in Article I, Section 8, gives Congress the power to exercise exclusive legislation over the District. In 1973, Congress passed the District of Columbia Home Rule Act, which gives the District's elected government some legislative authority similar to that of a state. The Act specifically states that the provisions of the federal laws of the United States do apply in the District, with certain exceptions. Federal law prohibits the possession, sale and distribution of marijuana. Because the District is not a state, the District's marijuana laws are subject to the exclusive legislative authority of Congress.