The Penner (also Pennar or Penneru) is a river of southern India. The Penner rises on the hill of Nandidrug in Kolar district of Karnataka state, and runs 560 km (350 miles) north and east through Andhra Pradesh state to empty into the Bay of Bengal. The watershed of the Penner and its tributaries covers part of the southern Deccan plateau, including most of the Rayalaseema region of Andhra Pradesh. and part of Karnataka. The Kolar plateau forms the divide between the Penner watershed and those of the Kaveri, Ponnaiyar, and Palar rivers to the south. The Penner drains the northern portion of the plateau, which includes parts of Kolar and Tumkur districts in Karnataka. The Krishna River and its tributaries drain the Deccan plateau to the west and north of the Penner's watershed, and the low Erramala hills forms the northern divide of the Penner basin. The upper watershed of the Penner includes Cuddapah District, central and eastern Anantapur District, the southern part of Kurnool District, northwestern Chittoor District. The main tributaries of the Penner are the Jayamangali, Kunderu and Sagileru from the north, and the Chitrabvati, Papaghni and Cheyyeru from the south. The Penner then flows east through a gap in the Eastern Ghats ranges onto the plain of Coastal Andhra, where it empties into the Bay of Bengal 15 km east of Nellore.

The upper basin of the Penner is largely made up of ancient Archean rocks, principally granite and schist. The lower basin is made up of young sediments, including laterite and recent alluvium.

The Penner basin has a dry tropical monsoon climate. The upper basin of the Penner is semi-arid, with summer temperatures of 25-46ºC and winter temperatures of 18 - 28ºC. Most of the rainfall is provided by the moisture-bearing southwest monsoon, which provides rain to most of India between June and September. The Penner basin lies in the rain shadow of the high Western Ghats ranges, which prevents much moisture from reaching the region. Post-monsoon cyclonic storms in the coastal region produce additional rain during September and October. The winter northeast monsoon, which provides rain between December and March, provides little rain to the upper basin and slightly more to the lower basin. It varies greatly from year to year in south India, sometimes bringing powerful cyclonic storms with flooding and high winds. The mean annual rainfall is 550 mm/year in Anantapur, and 900 mm/year in Nellore. The Penner basin suffered from a prolonged drought in the 1990's, which caused much misery among the regions farmers and generated political demands to build an aqueduct to bring water from the Krishna River to Rayalaseema.

The upper basin was formerly covered by tropical dry forest, thorn forest, and xeric shrublands. Most of the dry tropical forest has now disappeared, due to clearance for grazing and overharvesting the forests for timber and firewood, replaced by thorny shrublands. The remnant forests of the Deccan are largely deciduous, dropping their leaves in the dry winter and spring months. The East Deccan dry evergreen forests of Coastal Andhra were evergreen, but these forests have largely been reduced to tiny remnant pockets.

The estuary of the Penner river extends 7km upstream from the Bay of Bengal. Tidal influence and salt water extends further upstream during the November to June dry season. Coastal dunes as high as 7 meters form around the river mouth. Upputeru tidal creek, 15km in length, and Isakapalli lagoon, separated from the Bay of Bengal by the Isakapalli barrier island, 180 m long and up to 3 m high, form the main coastal wetlands. The small boat port of Krishnapatnam lies on Upputeru creek, and is planned to be developed into a deep-water port by 2006. Buckingham Canal, a navigable man-made waterway that runs just behind the coast, allows small boats from the Penner to get to Chennai in the south and the Krishna River delta to the north.