Naṣr ibn Sayyār al-Lāythi al-Kināni (Arabic: نصر بن سيار الليثي الكناني;‎ 663 – 9 December 748) was an Arab general and the last Umayyad governor of Khurasan in 738–748. Nasr played a distinguished role in the wars against the Turgesh, although he failed to decisively confront the rebellion of al-Harith ibn Surayj in its early stages. Although respected as a soldier and a statesman, he owed his appointment as governor more to his obscure tribal background, which rendered him dependent on the caliph. His tenure was nevertheless successful, as Nasr introduced long-overdue tax reforms that alleviated social tension and largely restored and stabilized Umayyad control in Transoxiana, which had been greatly reduced under the Turgesh onslaught. His last years were occupied by inter-tribal rivalries and uprisings, however, as the Umayyad Caliphate itself descended into a period of civil war. In 746 Nasr was driven from his capital by Ibn Surayj and Juday al-Kirmani, but returned after the latter fell out among themselves, resulting in Ibn Surayj's death. Preoccupied with this conflict, Nasr was unable to stop the outbreak and spread of the Abbasid Revolution, whose leader, Abu Muslim, exploited the situation to his advantage. Evicted from his province in early 748, he fled to Persia pursued by the Abbasid forces, where he died on 9 December 748.

Nasr was a military leader with long service and experience in Khurasan. As early as 705 he participated in a campaign along the upper Oxus River, led by Salih, the brother of Qutayba ibn Muslim, the general who had been tasked with subduing Transoxiana. For his service during this campaign, Nasr was awarded an entire village in this region. Despite the successes of Qutayba, much of Central Asia east of the Oxus remained outside effective Arab control; while garrisons had been established in places like Samarkand, Balkh, or Bukhara, the Umayyad Caliphate largely relied on cliental relationships with the multitude of local rulers, who became tributary to the Umayyads. In addition, clashes with the Chinese-backed Türgesh, the ambiguous policy followed regarding conversion of the native population (mass conversions would lessen the taxable population and hence the amount of tribute received) and increasing inter-Arab tribal factionalism weakened Umayyad control over the region and necessitated increased military activity.

In 724, Nasr is recorded as heading a Mudari army sent against Balkh, where restive Yamani troops refused to participate in the expedition against Ferghana that resulted in the disastrous 'Day of Thirst'. His troops, reinforced by men from the subject Hephthalite principality of Chaghaniyan, clashed with the Yaman at Baruqan and prevailed over them. This led to resentment towards his person among the Yaman, especially from those around Balkh; and during the governorship of the Yamani Asad ibn Abdallah al-Qasri, along with other Mudari leaders, Nasr fell into disfavour and was mistreated.

Nasr was one of the few Muslim leaders to distinguish himself in the disastrous Battle of the Defile in July 731. In 734 he was appointed as governor of Balkh, after arresting the previous governor. There he faced the rebellion of the local Khurasani troops under al-Harith ibn Surayj, who called for reforms in taxation and the ending of discrimination towards the native converts (mawali). Ibn Surayj marched on Balkh and took the city with only 4,000 followers, even though Nasr commanded 10,000 men. It is unclear from the sources whether the town was seized from Nasr, or whether it was captured in his absence and then successfully held against him. In any case, Nasr and his army remained passive for the remainder of the revolt; they did not aid the provincial capital, Merv, when the rebels attacked it, and this stance encouraged several local tribes to join the uprising. Eventually however the rebels were defeated by Juday al-Kirmani, with Ibn Surayj fleeing across the Oxus to the Türgesh.

In July 738, at the age of 74, Nasr was appointed as governor of Khurasan. Despite his age, he was widely respected both for his military record, his knowledge of the affairs of Khurasan and his abilities as a statesman. Julius Wellhausen wrote of him that "His age did not affect the freshness of his mind, as is testified not only by his deeds, but also by the verses in which he gave expression to his feelings till the very end of his life". However, in the climate of the times, his nomination owed more to his appropriate tribal affiliation than his personal qualities.

From the early days of the Muslim conquests, Arab armies were divided into regiments drawn from individual tribes or tribal confederations (butun or ʿashaʿir). Despite the fact that many of these groupings were recent creations, created for reasons of military efficiency rather than any common ancestry, they soon developed a strong and distinct identity. Eventually, and certainly by the beginning of the Umayyad period, this system progressed to the formation of ever-larger super-groupings, culminating in the two super-groups: the northern Arab Mudar or Qays, and the southern Arabs or Yaman, dominated by the Azd and Rabi'ah tribes. By the 8th century, this division had become firmly established across the Caliphate and was a source of constant internal instability, as the two groups formed in essence two rival political parties, jockeying for power and separated by a fierce hatred for each other. During the reign of Caliph Hisham ibn Abd al-Malik (r. 724–743), the Umayyad government appointed Mudari governors in Khurasan, except for Asad ibn Abdallah al-Qasri's tenure in 735–738. Nasr's appointment came four months after Asad's death. In the interim, the sources report variously that the province was run either by the Syrian general Ja'far ibn Hanzala al-Bahrani or by Asad's lieutenant Juday al-Kirmani. At any rate, the sources agree that al-Kirmani stood at the time as the most prominent man in Khurasan and should have been the clear choice for governor. His Yamani roots (he was the leader of the Azd in Khurasan), however, made him unpalatable to the caliph.

Nasr on the other hand, in addition to his other qualities, was a Mudari and married to a Tamimi wife. He would therefore be acceptable to the numerous Mudari element of the Khurasani army, which outnumbered the Yaman, but could also, as a local, help to reduce the Khurasani Arabs' discontent towards the Syria-centric Umayyad government. Nasr's own relatively obscure tribal background—from a non-noble family of the Layth tribe from Kinanah—also suited the caliph's purposes, as it meant that he lacked any local power base of his own. His position was thus heavily reliant on the Umayyad central government in Damascus, no doubt as Hisham intended: the caliph is reported to have remarked that he himself would be Nasr's tribal group. As a result, Nasr's rule throughout his tenure was not fully accepted by many Arab tribesmen: aside from the Yaman, who favoured their own candidate al-Kirmani and resented the shift in power back towards the Mudar, the Qays around Nishapur refused to support him, and even the Syrian contingent sided with his opponents. Nasr was hence mostly reliant on the support of his wife's powerful Tamim tribe, living around Merv. As long as he was supported by a strong central government, Nasr was able to keep his internal enemies in check, but in the troubles that followed Hisham's death in 743, that support vanished. In the event, Nasr would succeed in retaining his office for a decade, despite the turmoil that swept the Caliphate after 743. When Yazid III came to power in early 744, he initially ordered Nasr replaced. Nasr refused to accept this, and held on to the post, being eventually confirmed to it a few months later. After Marwan II's rise to power in December 744, he likewise affirmed Nasr's position.

Nasr gave his province an unprecedented period of good government, stability and prosperity, so that, in the words of the 9th-century historian al-Mada'ini, "Khurasan was built up as it had never been before". His major achievements during his tenure were the reform of the tax system and the restoration of Umayyad control over Transoxiana.