First round

The format of the competition changed from 1978: for the first time, 24 teams qualified, divided into six groups of four (1 through 6). The top two teams in each group advanced to the second round, where they split into four groups of three (A to D). The winners of each group advanced to the semi-finals. This was the only World Cup to be played under this format. The decision to expand from 16 to 24 teams came from FIFA to give the opportunity to more teams to participate, especially teams from North America, Africa and Asia. The first round was marked by a series of surprisingly strong showings by these supposedly weaker teams, although the more established football powers generally prevailed in advancing to the next stage. In Group 1, first-time participants Cameroon held both Poland and Italy to draws, and only failing to advance only on the basis of fewer goals scored than Italy. However, there was some controversy in the group. In the Peru-Cameroon game, Cameroonian striker Roger Milla had a goal disallowed for offside, but TV replays showed that Milla was clearly onside when he scored the goal. The decision contributed to Cameroon's early exit, as Italy would have been eliminated instead if Milla's goal stood. As a result, Poland and Italy qualified over Cameroon and Peru. Italian journalists and tifosi criticised their team for their uninspired performances that managed three draws; the squad was reeling from the recent Serie A scandal, where national players were suspended for match fixing and illegal betting. Paolo Rossi was singled out for his poor form, being described as a ghost wandering aimlessly over the field. The Azzurri decided on a press black-out from then on, with only coach Enzo Bearzot and captain Dino Zoff allowed to speak to the press. Group 2 saw one of the great World Cup upsets on the first day with the 2-1 victory of Algeria over reigning European Champion West Germany. This memorable game resulted in the controversial match between West Germany and Austria, their third and final match. As Algeria had already played their third match the day before, West Germany and Austria knew that a West German win by 1 or 2 goals would qualify them both, while a larger German victory would qualify Algeria over Austria, and a draw or an Austrian win would eliminate the Germans. The fourth team in the group, Chile, were eliminated regardless of the outcome. After 10 minutes of furious attack, West Germany succeeded in scoring through a goal by Horst Hrubesch. After the goal was scored, the two German-speaking teams went into an unspoken agreement and just kicked the ball around aimlessly for the rest of the match. Chants of "Fuera, fuera" ("Out, out") were screamed by the appalled Spanish crowd, while angry Algerian supporters waved banknotes at the players. This sham performance was widely deplored, even by the German and Austrian fans who had hoped for a hot rematch of the 1978 FIFA World Cup match in which Austria had beaten West Germany. One German fan was so upset by his team's display that he burned his German flag in disgust. As a result of the outcome, FIFA introduced a revised qualification system at subsequent World Cups in which the final two games in each group were played simultaneously. Group 3, where the opening ceremonies and first match of the tournament took place, yielded a major upset with Belgium beating defending champions Argentina 1–0. The Camp Nou stadium was the home of Barcelona, and many fans had wanted to see the club's new signing, Argentinian star Diego Maradona, who did not perform to expectations. Both Belgium and Argentina ultimately advanced at the expense of Hungary and El Salvador despite Hungary's 10-1 win over the Central American nation — which, with a total of 11 goals, is the second highest scoreline in a World Cup game (as well as Brazil's 6-5 victory over Poland in the 1938 tournament and Hungary's 8-3 victory over West Germany in the 1954 tournament), second to the 12-goal Austria-Switzerland (7-5) match in the 1954 tournament.
Group 4 opened at record speed with England midfielder Bryan Robson's goal against France after only 27 seconds of play. England won the game 3-1 and qualified along with France over Czechoslovakia and Kuwait, though the tiny Gulf emirate created yet another sensation by holding Czechoslovakia to a 1-1 draw. This group was also the stage of a farcical incident during the game between Kuwait and France. As Les Bleus were leading 3-1, France midfielder Alain Giresse scored a goal vehemently contested by the Kuwait team, who had stopped play after hearing a piercing whistle from the stands, as the French player was in a suspicious, arguably offside position, which they thought had come from Soviet referee Stupar. Play had not yet resumed when Sheikh Fahid Al-Ahmad Al-Sabah, brother of the Kuwaiti Emir and president of the Kuwaiti Football Association, rushed onto the pitch to give the referee a vociferous piece of his mind. The visibly shaken Stupar countermanded his initial decision and disallowed the goal to the understandable fury of the French. Maxime Bossis scored another valid goal a few minutes later and France won 4-1. Stupar lost his international refereeing credentials due to this incident, and al-Sabah received a $10,000 fine. In Group 5, the uprising of the minnows continued with Honduras holding hosts Spain to a 1-1 draw, then became a full-fledged revolution as unfancied Northern Ireland won the group outright, eliminating Yugoslavia and beating Spain 1-0 on its home ground in the process, the result was even more impressive as Northern Ireland had to play the majority of the second half with ten men after Mal Donaghy was dismissed. The home team showed that they were not serious contenders this year, barely scraping by thanks to yet another controversial penalty in its 2-1 victory over Yugoslavia. All eyes were on Brazil in Group 6. Around Zico, Sócrates, Falcão, Éder and fellows, the rejuvenated auriverde boasted an offensive firepower that promised a return to the glory days of 1970. The tri-campeão lived up to all expectations, beating an unexpectedly strong Soviet side 2-1 in a very entertaining first match thanks to a 20 metre Éder goal two minutes from time, then Scotland and New Zealand with four goals each. The promising Soviets took the group's other qualifying berth at the expense of the Scots, Rinat Dasayev emerging as a worthy successor to all-time legend Lev Yashin in the Soviet goal with a rarely talked about save from Joe Jordan's header. The ball was virtually on the line when Dasayev flicked it around the post with his finger at the last possible moment, much to the disbelief of the Scots and watching spectators. New Zealand earned international respect in their World Cup opener against Scotland, cutting the Scots' 3-0 lead to 3-2 before conceding two more goals late in the game.