The city emerged as a trade centre in second half of the 19th century. During World War II it was a prime objective for conquest by the Imperial Japanese forces during 1942–43 as a staging point and air base to cut off Australia from Southeast Asia and the Americas.

In 2000 it had a population of 254,158. As of 2011 it has a population of 364,145, giving it an annual growth rate of 2.1% over a nine-year period. The place where the city was founded has been inhabited by the Motu-Koitabu people for centuries. The first European to see it was Captain John Moresby in 1873. It was named in honour of his father, Admiral Sir Fairfax Moresby.

According to a survey of world cities by the Intelligence Unit of The Economist, Port Moresby is one of the world's least livable cities (ranked 139 of 140 cities rated).

Although Port Moresby is surrounded by Central Province, of which it is also the capital, it is not part of that province, but forms the National Capital District.

Port Moresby has a tropical wet and dry climate with relatively constant temperatures throughout the year. The wet season starts in December and ends in May; the dry season covers the remaining six months. Port Moresby's average yearly rainfall is just over 1,000 mm (39.4 in). Average daily high temperatures range from 28 to 32 °C (82 to 90 °F) depending on time of year, while the average low temperature shows very little seasonal variation, hovering around the 26 °C (79 °F) mark. It tends to be slightly cooler in the city during the dry season.

Port Moresby refers to both the urbanised area of the National Capital District and more specifically to the main business area, known locally as "Town".

 Since the 1990s the original town centre has ceased to have restaurants and night life, though it is very successful and prosperous looking as an office centre; the affluent housing region north of downtown along and up from the coach remains so, though there are now few modest residential houses, most of which are replaced with substantial mansions and apartment buildings.

 The suburb of Boroko, once the commercial heart of Port Moresby, is currently very idle, with many former shopping buildings now simply empty; to the west of there, however, is full of high rises, shopping centres and affluent housing. 

Other neighbourhoods of Port Moresby include: Koki, with its popular fresh produce market, Newtown, Konedobu, Kaevaga, Badili, Gabutu, Kila Kila, Matirogo, Three Mile, Kaugere, Sabama, Korobosea, Four Mile, Hohola, Hohola North, Boroko, once a major shopping area, Gordons, Gordons North, Erima, Saraga, Waigani, Morata and Gerehu. There are also villages like Hanuabada, the largest village in Papua New Guinea. 

All parts of Port Moresby have serious security problems with violent attacks by criminals, causing fences and walls to be considered necessary around houses and apartment buildings and security guards to be widely employed. The UN Global Compact Cities Programme, using a method called Circles of Sustainability has assessed the urban security of Port Moresby as 'critical'.

Port Moresby is served within the city by buses and privately owned taxis. Flights are vital for transport about the country, highways not being widely available. Port Moresby is served by Jacksons International Airport, the biggest international airport and Papua New Guinea Defence Force Air Wing base in the country. Air Niugini and Airlines PNG both conduct regular domestic and international services from the airport, while Virgin Australia flies to Brisbane. Jacksons has international flights to Brisbane, Cairns, Cebu, Sydney, Honiara, Nadi, Manila, Singapore, Hong Kong and Tokyo.

As the national highway system is not fully linked, there are many internal flights to other towns, e.g. Madang, which cannot be reached by ground transport, such as minibuses, known locally as PMVs (Public Motor Vehicles).