The rectum (pl.: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult human rectum is about 12 centimetres (4.7 in) long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used. Its diameter is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal, which is about 4 centimetres (1.6 in) long, before the gastrointestinal tract terminates at the anal verge. The word rectum comes from the Latin rēctum intestīnum, meaning straight intestine.

The human rectum is a part of the lower gastrointestinal tract. The rectum is a continuation of the sigmoid colon, and connects to the anus. The rectum follows the shape of the sacrum and ends in an expanded section called an ampulla where feces is stored before its release via the anal canal. An ampulla (from Latin bottle) is a cavity, or the dilated end of a duct, shaped like a Roman ampulla. The rectum joins with the sigmoid colon at the level of S3, and joins with the anal canal as it passes through the pelvic floor muscles.

Unlike other portions of the colon, the rectum does not have distinct taeniae coli. The taeniae blend with one another in the sigmoid colon five centimeters above the rectum, becoming a singular longitudinal muscle that surrounds the rectum on all sides for its entire length.

The blood supply of the rectum changes between the top and bottom portions. The top two thirds is supplied by the superior rectal artery. The lower third is supplied by the middle and inferior rectal arteries.

The superior rectal artery is a single artery that is a continuation of the inferior mesenteric artery, when it crosses the pelvic brim. It enters the mesorectum at the level of S3, and then splits into two branches, which run at the lateral back part of the rectum, and then the sides of the rectum. These then end in branches in the submucosa, which join with (anastamose) with branches of the middle and inferior rectal arteries.

The microanatomy of the wall of the rectum is similar to the rest of the gastrointestinal tract; namely, that it possesses a mucosa with a lining of a single layer of column-shaped cells with mucus-secreting goblet cells interspersed, resting on a lamina propria, with a layer of smooth muscle called muscularis mucosa. This sits on an underlying submucosa of connective tissue, surrounded by a muscularis propria of two bands of muscle, an inner circular band and an outer longitudinal one. There are a higher concentration of goblet cells in the rectal mucosa than other parts of the gastrointestinal tract.

The lining of the rectum changes sharply at the line where the rectum meets the anus. Here, the lining changes from the column-shaped cells of the rectum to multiple layers of flat cells.

The rectum acts as a temporary storage site for feces. The rectum receives fecal material from the descending colon, transmitted through regular muscle contractions called peristalsis. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to pass feces, a process called defecation.