Ilissos Basilica was the basilica built the east of the Olympieion. It is three-aisled with a transept, a narthex and an atrium. The martyrium of Hagios Leonidis, bishop of Athens, lies in the northern wall. It is a 4th century building anterior to the basilica, which is dated to the first half of the 5th century based on the floor mosaics.
Despite their fragmentary state of preservation the mosaics of Ilissos Basilica show decoration of exceptional quality. The surface is decorated by bands of interlace which divide it up into individual sections, each of which contains one of a variety of decorative motifs, combining Roman and Christian traditions. The former is represented by features such as: interlace in the form of chains ("guilloche"), stylized round flowers (rosettes), trailing ivy, motifs resembling fish scales and water birds. Among the Christian motifs are vine scroll from which hang bunches of grapes and vine leaves (symbol of the Christian Paradise), wreaths of laurel leaves (a well-known symbol of victory from Roman times), small crosses and other geometrical and plant motifs. The decorative elements are distinguished by their extensive range of colors and the charm of their execution. The mosaic uses white, black, deep red, orange, grey, a pale violet, brown, yellow, pink, blue and green. The drawing, especially of the vine, shows a light touch and great care has been paid to arranging the tesserae to follow the outlines.