The name Rajgir, literally meaning "royal mountain", comes from the historic Rājagṛiha (Sanskrit Rājagṛha, Pali: Rajagaha), meaning "house of the king" or "royal house". It has also historically been known as Vasumati, Brhadrathapura, Grivraja/Girivraja and Kusagrapura. Girivraja, a name given in the Rāmāyaṇa as well as in Pali texts (as Giribajja), is explained by Buddhaghosa as meaning "an enclosure of hills". "Vasumatī" is a name given in the Rāmāyaṇa because of the city's legendary founding by Vasu, the fourth son of Brahmā. The Mahābhārata, on the other hand, attributes the city's founding to Bṛhadratha, and accordingly it calls the city Bārhadrathapura. The name Kuśāgrapura is given in the Jinaprabhasūri and also (in Chinese) by Xuanzang; this name means "the place of superior reed grass".

The modern town of Rajgir lies just to the north of the Rajgir Hills, an isolated pair of parallel ridges that rise sharply up from the flat surrounding plains. The ancient site of Rajgir was located in the narrow valley between the two sets of hills. Historically, Rajgir was thought of as lying between five hills, but different texts give them different names (the Mahābhārata even gives two different lists of five names in the same chapter), making it difficult to identify the five hills with their modern counterparts. The modern names are Vaibhara, Vipula, Chaṭa, Shaila, Udaya, Sona, and Ratnagiri. Of these, Ratnagiri is the highest, at about 305 m.

Rajgir is known for its hot springs, which have held religious significance since ancient times. The best-known today is Brahmakuṇḍa, and there are several other springs in this area whose waters merge to form a stream. Another prominent spring is the Sūryakuṇḍa, which is part of a cluster of springs on the north side of the hills. Sūryakuṇḍa's water is stagnant and murky and a favored habitat by frogs. Another major water feature in the area is the Panchane River (from Pañcānanā, "having five mouths"), which flows to the east of Rajgir.

Rajgir has a monsoon climate, with an annual rainfall of about 113 cm. Rainfall peaks in July and August.
• Summer temperature: maximum 44 °C (111.2 °F), minimum 20 °C (68 °F)
• Winter temperature: maximum 28 °C (82.4 °F), minimum 6 °C (42.8 °F)
• Rainfall: 1,860 mm (mid-June to mid-September)
• Dry/warm season: March to October
The landscape of Rajgir or Pant WLS is uneven terrain enclosed by five hills; Ratnagiri, Vipulgiri, Vaibhagiri, Songiri and Udaygiri. It is situated in Nalanda Forest Division covering an area of 35.84 km under the Nalanda district administration. This wildlife sanctuary, notified in 1978, represents a remnant patch of forests nestled in the Rajgir hills within the south Gangetic Plain.

It is home to a number of wild animals including: mammals – blue bull (Boselaphus tragocamelus), chital or spotted deer (Axis axis), Indian crested porcupine (Hystrix indica), small Indian civet (Viverricula indica), jungle cat (Felis chaus); birds – painted spurfowl (Galloperdix lunulata), Eurasian thick knee (Burhinus oedicnemus), painted sandgrouse (Pterocles indicus); reptiles and amphibians – Bengal monitor (Varanus bengalensis), Indian bullfrog (Hoplobatrachus tigerinus), Jerdon's bullfrog (Hoplobatrachus crassus), ornate narrow-mouthed frog (Microhyla ornata), and Indian tree frog (Polypedates maculatus).

According to 2011 Indian Census, Rajgir had a total population of 41,587, of which 21,869 were males and 19,718 were females. The population within the age group of 0 to 6 years was 6,922. The total number of literates in Rajgir was 24,121, which constituted 58.0% of the population with male literacy of 65.4% and female literacy of 49.8%. The effective literacy rate of the 7+ population of Rajgir was 69.6%, of which the male literacy rate was 78.1% and the female literacy rate was 60.1%. The Scheduled Castes and Scheduled Tribes population was 11,724 and 42 respectively. Rajgir had 7030 households in 2011.
The town is also notable in Jainism and Buddhism. It was the birthplace of the 20th Jain Tirthankar Munisuvrata, and is closely associated with the Mahavira and Gautama Buddha. Both Mahavira and Buddha taught their beliefs in Rajgir during the 6th and 5th century BC, and the Buddha was offered a forest monastery here by the king Bimbisara. As such, the city of Rajgir became one of the Buddha's most important preaching locations.