The Robert A. Grant Federal Building and U.S. Courthouse is located on the southeast corner of the intersection of Main and Jefferson Streets in downtown South Bend, Indiana. The rectangular lot is bordered by Main Street on the west, Jefferson Street on the north, and city alleys on the east and south. Nearly the entire site is covered by the building. The building, with a full basement, three floors, and a mezzanine, has two primary elevations on the north and west. The building is designed in the Art Moderne or Art Deco style. The primary elevations have low relief and consist of pilasters alternating with bays of vertical strips of windows. The intercolumnar rhythm is flanked by slightly projecting corner pavilions. Entrances are located within these pavilions at either end of the west facade. The main block of the building is terminated by a continuous carved cornice. Above the cornice line is another floor set back from the face of the building.
The exterior walls are constructed with brick masonry faced with Indiana limestone and Vermont granite veneer, except for the third floor interior lightwells which have brick cladding. The primary elevations feature decorative elements of cast bronze. The original doors and windows have been replaced with new aluminum units. The building is covered with flat roof surfaces with the exception of the roof over the third floor courtroom which is a hopped and covered with standing seam copper.
The primary elevations of the building face west and north. The elevations have gray granite base and are clad with a smooth limestone veneer. The elevations are divided into ten bays of vertical window fenestrations set between nearly solid, slightly projecting pavilions at the ends. The building's two main entrances, identical to each other, are located in the end pavilions on the west elevation. Oversized carved stone surrounds with modified egg-and-dart motifs contain pairs of replacement aluminum entrance doors set into bronze frames which include ornate transom windows. Above the entrance doors are glazed transoms with bronze grilles. The portions of the elevations between the end pavilions consist of Doric pilasters separating vertical groups of windows. The vertical window bands consist of replacement aluminum triple casement sash with trisected transoms set in the original bronze frames. To create the vertical banding effect, bronze spandrels with varying designs (grains, flowers, abstracts) were placed at the floor levels. The spandrels over the entrance doorways feature a spread eagle motif. A cornice, composed of a modified Doric entablature with guttae details sits atop the two-story pilasters and contains the incised letters "UNITED STATES OF AMERICA" on the west elevation. The third floor of the building sits back slightly from the west and north faces of the building, and are divided into twelve bays of fenestration with the same replacement aluminum triple casement sash with trisected transoms. A simplified cornice with abstract carvings top the elevation. Bronze letters reading "ROBERT A. GRANT FEDERAL BUILDING AND UNITED STATES COURTHOUSE" are attached to the limestone adjacent to each entrance. The building's cornerstone with incised lettering is adjacent to the north entrance.
The details and materials of the end pavilion wrap around to the east and south elevations. The remainder of these elevations, though clad with the same granite and matching limestone, are much simpler in appearance and are devoid of the decorative carved details. Window openings contain either single or paired casement sash with corresponding transoms. Several door and window openings have been infilled with metal louvers of limestone blocks. The main feature of the east elevation is the raised loading dock platform with its projecting canopy. An overhead garage door, also located on the east elevation, provides access to the basement parking court.
The building features two highly significant interior spaces: a two-story, barrel-vaulted postal lobby trimmed in marble and polished brass on the first floor, and a two-story courtroom trimmed with walnut paneling on the third floor. Most of the remainder of the interior spaces have been substantially renovated over the years.