Avinguda Diagonal (Catalan: [əβiŋˈɡuðə ði.əɣuˈnal]; Spanish: Avenida Diagonal; transl. 'Diagonal Avenue') is the name of one of Barcelona's broadest and most important avenues. It cuts the city in two, diagonally with respect to the grid pattern of the surrounding streets, hence the name.

It was originally projected by engineer and urban planner Ildefons Cerdà as one of the city's wide avenues, which along with Avinguda Meridiana would cut the rationalist grid he designed for Eixample (Catalan for 'Expansion'). Both would meet at Plaça de les Glòries Catalanes, which Cerdà envisioned as the new city centre. However, Plaça Catalunya, equally a new addition to the city of Barcelona, and connecting Ciutat Vella and Eixample, and therefore occupying a more privileged position in the urban area, would finally become the centre. Avinguda Diagonal remains to this day a much-transited avenue and many companies and hotels use it as a privileged location, as can be seen in its architecture.

The avenue starts in the Les Corts district on the western edge of the city and runs to the Sant Martí district on the eastern edge. To its west, it connects with the Lleida-Madrid highway and Ronda de Dalt in the neighbouring municipality of Esplugues de Llobregat. To its east, it meets the Ronda del Litoral on the border with the municipality of Sant Adrià de Besòs. It is consistently 50 metres (160 ft) wide and about 11 kilometres (6.8 mi) long.

The different regimes that held power in Catalonia and Spain during the 20th century sought to change the city's street names, and Avinguda Diagonal was permitted no exception. It has been known under the following names:

The name "Diagonal" has always prevailed in popular usage.

Ildefons Cerdà's so-called Pla Cerdà was not totally successful in transforming Barcelona's urban reality, as only parts of it were finally approved. The construction of Avinguda Diagonal is one of the projects it entailed that became reality, when a Royal Decree from Queen Isabella II of Spain and Leopoldo O'Donnell's Spanish government in Madrid allowed him to start the construction of the avenue in 1859. The city council of Barcelona had previously requested the approval of Antoni Rovira i Trias's alternative project instead, which had been rejected.

After the completion of its central section, from the current Plaça de Francesc Macià towards Glòries, it soon became one of Barcelona's most popular avenues and an ideal place for the Catalan aristocrats and bourgeoisie to exhibit their carriages. Francesc Cambó, leader of Lliga Regionalista proposed the construction of a new palace for the then monarch Alfonso XIII in 1919 (the royal palace in Ciutat Vella had been destroyed in the fire of 1875).

Avinguda Diagonal suffered several attacks by the Basque separatist organisation ETA in 2000. On 2 November, a car bomb exploded injuring a security officer and a municipal officer. A few days later, on 21 November, a car used by the killers of Ernest Lluch exploded in the middle of the avenue. On 20 December a municipal officer was shot dead.