Senwosret I (Sesostris), the son and successor to Amenemhet I, built his pyramid at the southern end of the site at Lisht, known today as the South Lisht pyramid. It was investigated, like the North Lisht pyramid by Maspero in 1882 when he discovered the pyramid's owner from objects he found, bearing the king's name.

Senwosret's monument is larger than that of his father, though built basically to the same plan. However, Senwosret's architects invented a new technique which was used by Senwosret II at el-Lahun and throughout the Middle Kingdom, in which a core of limestone block walls radiated from the centre of the structure, and the spaces filled with unfired mudbricks and debris before being covered with a Tura limestone casing. In theory this made for a stronger structure. Some of the casing is still preserved and one of the walls of the framework is visible, but the pyramid today is little more than a low mound.

The entrance opened in a pavement at ground level on the north side of the structure, with an entrance chapel covering it. Fragments of coloured reliefs have been found from the chapel, as well as fragments of an altar and an alabaster false door stela. There were also water spouts in the shape of lions' heads (similar to those found in later temples) to drain water away from the roof. The entrance passage sloped downwards then turned towards the south-east but like Amenemhet's burial chamber this is now under water. Another tunnel was dug below the entrance passage to facilitate the transporting of materials to the burial chamber, which was presumably blocked up on completion of the pyramid. It is believed that the burial chamber was robbed shortly after it had been sealed, though Maspero found remains of the king's funerary goods in the robber's tunnel.

The complex is surrounded by a double perimeter wall, the first enclosing part of the king's mortuary temple on the eastern side and a small satellite pyramid at the south-east corner. The inside of the first perimeter wall was uniquely decorated with panels of reliefs with the king's names and images of fertility gods. Senwosret's funerary temple is almost completely destroyed, although a little better preserved than that of his father, making it easier for archaeologists to reconstruct the plan. This conformed to funerary temples of Dynasty VI, with remains of a courtyard and portico which had once had 24 pillars and an offering hall flanked by store-rooms. During excavations by Gautier in 1894 in the area of the mortuary temple, ten life-sized seated limestone statues of Senwosret were found in a pit between two subsidiary temples (now in Cairo Museum).

Nine more secondary pyramids for female members of the king's family were found inside the outer mudbrick enclosure wall. Seven of the small pyramids had their own funerary temple and enclosure wall and the last two, on the northern side shared an enclosure. Only two of the names of royal ladies have so far been recovered, those of Nereru (or Nofret), wife (and sister) of Senwosret and Itayket (or Itaksiet) who was probably Senwosret's daughter.

An open causeway connected the mortuary temple to the valley temple which has not yet been precisely located. Today the causeway's walls which were decorated with reliefs, are still visible in places.