The interior chamber of the Taj Mahal reaches far beyond traditional decorative elements. The inlay work is not pietra dura, but a lapidary of precious and semiprecious gemstones.[22] The inner chamber is an octagon with the design allowing for entry from each face, although only the door facing the garden to the south is used. The interior walls are about 25 metres (82 ft) high and are topped by a "false" interior dome decorated with a sun motif. Eight pishtaq arches define the space at ground level and, as with the exterior, each lower pishtaq is crowned by a second pishtaq about midway up the wall.[23] The four central upper arches form balconies or viewing areas, and each balcony's exterior window has an intricate screen or jali cut from marble. In addition to the light from the balcony screens, light enters through roof openings covered by chattris at the corners. The octagonal marble screen or jali bordering the cenotaphs is made from eight marble panels carved through with intricate pierce work. The remaining surfaces are inlaid in delicate detail with semi-precious stones forming twining vines, fruits and flowers. Each chamber wall is highly decorated with dado bas-relief, intricate lapidary inlay and refined calligraphy panels which reflect, in miniature detail, the design elements seen throughout the exterior of the complex.

The complex is set around a large 300-metre (980 ft) square charbagh or Mughal garden. The garden uses raised pathways that divide each of the four quarters of the garden into 16 sunken parterres or flowerbeds. Halfway between the tomb and gateway in the centre of the garden is a raised marble water tank with a reflecting pool positioned on a north-south axis to reflect the image of the mausoleum. The raised marble water tank is called al Hawd al-Kawthar in reference to the "Tank of Abundance" promised to Muhammad.

Elsewhere, the garden is laid out with avenues of trees and fountains. The charbagh garden, a design inspired by Persian gardens, was introduced to India by Babur, the first Mughal emperor. It symbolises the four flowing rivers of Jannah (Paradise) and reflects the Paradise garden derived from the Persian paridaeza, meaning 'walled garden'. In mystic Islamic texts of the Mughal period, Paradise is described as an ideal garden of abundance with four rivers flowing from a central spring or mountain, separating the garden into north, west, south and east.

Most Mughal charbaghs are rectangular with a tomb or pavilion in the centre. The Taj Mahal garden is unusual in that the main element, the tomb, is located at the end of the garden. With the discovery of Mahtab Bagh or "Moonlight Garden" on the other side of the Yamuna, the interpretation of the Archaeological Survey of India is that the Yamuna river itself was incorporated into the garden's design and was meant to be seen as one of the rivers of Paradise. Similarities in layout and architectural features with the Shalimar Gardens suggests both gardens may have been designed by the same architect, Ali Mardan. Early accounts of the garden describe its profusion of vegetation, including abundant roses, daffodils, and fruit trees. As the Mughal Empire declined, the Taj Mahal and its gardens also declined. By the end of the 19th century, the British Empire controlled more than three-fifths of India, and assumed management of the Taj Mahal. They changed the landscaping to their liking which more closely resembled the formal lawns of London.

The exterior decorations of the Taj Mahal are among the finest in Mughal architecture. As the surface area changes, the decorations are refined proportionally. The decorative elements were created by applying paint, stucco, stone inlays or carvings. In line with the Islamic prohibition against the use of anthropomorphic forms, the decorative elements can be grouped into either calligraphy, abstract forms or vegetative motifs. Throughout the complex are passages from the Qur'an that comprise some of the decorative elements. Recent scholarship suggests that the passages were chosen by Amanat Khan.

The calligraphy on the Great Gate reads "O Soul, thou art at rest. Return to the Lord at peace with Him, and He at peace with you." The calligraphy was created in 1609 by a calligrapher named Abdul Haq. Shah Jahan conferred the title of "Amanat Khan" upon him as a reward for his "dazzling virtuosity". Near the lines from the Qur'an at the base of the interior dome is the inscription, "Written by the insignificant being, Amanat Khan Shirazi." Much of the calligraphy is composed of florid thuluth script made of jasper or black marble inlaid in white marble panels. Higher panels are written in slightly larger script to reduce the skewing effect when viewed from below. The calligraphy found on the marble cenotaphs in the tomb is particularly detailed and delicate.

Abstract forms are used throughout, especially in the plinth, minarets, gateway, mosque, jawab and, to a lesser extent, on the surfaces of the tomb. The domes and vaults of the sandstone buildings are worked with tracery of incised painting to create elaborate geometric forms. Herringbone inlays define the space between many of the adjoining elements. White inlays are used in sandstone buildings, and dark or black inlays on the white marbles. Mortared areas of the marble buildings have been stained or painted in a contrasting colour which creates a complex array of geometric patterns. Floors and walkways use contrasting tiles or blocks in tessellation patterns.

On the lower walls of the tomb are white marble dados sculpted with realistic bas relief depictions of flowers and vines. The marble has been polished to emphasise the exquisite detailing of the carvings. The dado frames and archway spandrels have been decorated with pietra dura inlays of highly stylised, almost geometric vines, flowers and fruits. The inlay stones are of yellow marble, jasper and jade, polished and levelled to the surface of the walls.