The province is set in karstic limestone mountains and valleys.
Mountains and forests comprise 80% of the province's area.
The province measures approximately 123 kilometres from north to south and 126 kilometres from west to east.
The average altitude of the province is 252 metres (827 ft) above sea level.
The lowest point in the province is 20 metres (66 ft) towards the south of Hữu Lũng District and the highest point is at Mount Mẫu Sơn, which is 1,541 metres (5,056 ft).
Mẫu Sơn is to the east of the town of Lạng Sơn by 30 kilometres (19 mi), and is surrounded by a series of small peaks; snow sometimes falls on these peaks in winter.
The Bac Son Mountains are located in the province and are calcareous in nature.
Fertile valleys are framed by high mountain ridges, creating a scenic landscape.

The north of the province adjoins Cao Bằng Province.
The 243 kilometres (151 mi) Kỳ or Kyu River flows through Lạng Sơn town and alluvial plains are formed in the valley that is surrounded by high mountains of about 1,000 metres (3,300 ft) height.
The Kỳ has a basin area of 6,660 square kilometres (2,570 sq mi), and has its source in the mountainous area of Bắc Xa at an altitude of 1,166 metres (3,825 ft) in the district of Đình Lập.
The river is part of the Tây Giang River basin in China.
The Bản Thín River, a tributary of the Kỳ Cùng, which is 52 metres (171 ft) long with a catchment area of 320 square kilometres (120 sq mi), has its source in a mountainous region in Guangxi in China, and empties into the Kỳ Cùng in the commune of Khuất Xá in Lộc Bình Province.
Other tributaries of the Kỳ include the Bắc Giang River, which is 114 km long with a catchment area of 2,670 square kilometres (1,030 sq mi) and the Bắc Khê River, which is 54 kilometres (34 mi) long with a catchment area of 801 square kilometres (309 sq mi).
Other rivers of note in the province include the Thương Là River, which is the second largest in the province, sourced from the Na Pa Phước range in the district of Chi Lăng and is 157 km long with a basin area of 6,640 square kilometres (2,560 sq mi), the Hoá Độ River, which is 47 kilometres (29 mi) in length with a catchment area of 385 square kilometres (149 sq mi) and the Trung River, which is 35 km in length with a catchment area of1,270 square kilometres (490 sq mi).

Lạng Sơn has two international border crossings.
The most accessed is the Friendship Gate called the Hữu Nghị Quan crossing at Đồng Đăng connecting to Pingxiang town in China.
This is the historical first land link in the north between China and Vietnam that connects Lạng Sơn and Guangxi, China.
The border is open daily, from 07:00 to 17:00, and involves a walk of 500 m through the no man's territory between Vietnam and China.
There is an international train service, an express route, opened in 1996 from Hanoi to Beijing (China), which operates twice a week on Tuesday and Friday that passes through Lạng Sơn Town and Đồng Đăng through this gate which has three-hour stop at the border town to complete formalities of entry from one country to the other.
Passengers are not allowed to board the train at any intermediate station between Hanoi and Beijing.

Lạng Sơn is 155 kilometres (96 mi) to the northwest of Hanoi and National Highways 1 and 1 A and passes the Chi Lăng pass (the site of  Lê Lợi's victory over 100,000 Ming invaders from China in 1427) and Bắc Giang on National Highway 1A. Lạng Sơn is 135 km from Cao Bằng on the National Highway no 4.

Lạng Sơn Province has an average annual temperature of 17–22 °C and an average annual rainfall of 1,200–1,600 millimetres (47–63 in).
The average temperature in the summer is 28.5 °C (83.3 °F) and 12–13 °C (54–55 °F) in the winter, It has an average humidity of 80–85% and an average of 1600 hours of sunlight a year.
The province is set in karstic limestone mountains and valleys. Mountains and forests comprise 80% of the province's area. The province measures approximately 123 kilometres from north to south and 126 kilometres from west to east. The average altitude of the province is 252 metres (827 ft) above sea level. The lowest point in the province is 20 metres (66 ft) towards the south of Hữu Lũng District and the highest point is at Mount Mẫu Sơn, which is 1,541 metres (5,056 ft). Mẫu Sơn is to the east of the town of Lạng Sơn by 30 kilometres (19 mi), and is surrounded by a series of small peaks; snow sometimes falls on these peaks in winter. The Bac Son Mountains are located in the province and are calcareous in nature. Fertile valleys are framed by high mountain ridges, creating a scenic landscape.
The north of the province adjoins Cao Bằng Province. The 243 kilometres (151 mi) Kỳ or Kyu River flows through Lạng Sơn town and alluvial plains are formed in the valley that is surrounded by high mountains of about 1,000 metres (3,300 ft) height. The Kỳ has a basin area of 6,660 square kilometres (2,570 sq mi), and has its source in the mountainous area of Bắc Xa at an altitude of 1,166 metres (3,825 ft) in the district of Đình Lập. The river is part of the Tây Giang River basin in China. The Bản Thín River, a tributary of the Kỳ Cùng, which is 52 metres (171 ft) long with a catchment area of 320 square kilometres (120 sq mi), has its source in a mountainous region in Guangxi in China, and empties into the Kỳ Cùng in the commune of Khuất Xá in Lộc Bình Province. Other tributaries of the Kỳ include the Bắc Giang River, which is 114 km long with a catchment area of 2,670 square kilometres (1,030 sq mi) and the Bắc Khê River, which is 54 kilometres (34 mi) long with a catchment area of 801 square kilometres (309 sq mi). Other rivers of note in the province include the Thương Là River, which is the second largest in the province, sourced from the Na Pa Phước range in the district of Chi Lăng and is 157 km long with a basin area of 6,640 square kilometres (2,560 sq mi), the Hoá Độ River, which is 47 kilometres (29 mi) in length with a catchment area of 385 square kilometres (149 sq mi) and the Trung River, which is 35 km in length with a catchment area of1,270 square kilometres (490 sq mi).
Lạng Sơn has two international border crossings. The most accessed is the Friendship Gate called the Hữu Nghị Quan crossing at Đồng Đăng connecting to Pingxiang town in China. This is the historical first land link in the north between China and Vietnam that connects Lạng Sơn and Guangxi, China. The border is open daily, from 07:00 to 17:00, and involves a walk of 500 m through the no man's territory between Vietnam and China. There is an international train service, an express route, opened in 1996 from Hanoi to Beijing (China), which operates twice a week on Tuesday and Friday that passes through Lạng Sơn Town and Đồng Đăng through this gate which has three-hour stop at the border town to complete formalities of entry from one country to the other. Passengers are not allowed to board the train at any intermediate station between Hanoi and Beijing.
Lạng Sơn is 155 kilometres (96 mi) to the northwest of Hanoi and National Highways 1 and 1 A and passes the Chi Lăng pass (the site of  Lê Lợi's victory over 100,000 Ming invaders from China in 1427) and Bắc Giang on National Highway 1A. Lạng Sơn is 135 km from Cao Bằng on the National Highway no 4.
Lạng Sơn Province has an average annual temperature of 17–22 °C and an average annual rainfall of 1,200–1,600 millimetres (47–63 in). The average temperature in the summer is 28.5 °C (83.3 °F) and 12–13 °C (54–55 °F) in the winter, It has an average humidity of 80–85% and an average of 1600 hours of sunlight a year.
Lạng Sơn Province