Naturalization (or naturalisation) is the acquisition of citizenship and nationality by somebody who was not a citizen of that country at the time of birth.

In general, basic requirements for naturalization are that the applicant hold a legal status as a full-time resident for a minimum period of time and that the applicant promise to obey and uphold that country's laws, to which an oath or pledge of allegiance is sometimes added. Some countries also require that a naturalized national must renounce any other citizenship that they currently hold, forbidding dual citizenship, but whether this renunciation actually causes loss of the person's original citizenship will again depend on the laws of the countries involved.

Serginho "Sergio" van Dijk (born 6 August 1982) is a Dutch football player, who currently plays for Adelaide United in the A-League. He was the top goalscorer in the 2010–11 A-League regular season and has made a name for himself in Australian soccer for his prolific scoring tally.

He was named after the Brazilian star, Serginho Chulapa, who played for Brazil during the 1982 World Cup. Van Dijk's mother was a big fan of Brazil at the time. Van Dijk is of Indo (Dutch-Indonesian) parentage.

Van Dijk started his career at local club LTC in Assen before he was drafted in the FC Groningen youth-academy. In the 2000/01 season he played two matches (one goal) for Groningen in the Eerste Divisie helping them to gain promotion. In the Eredivisie he did not play for Groningen and he moved back to the Eerste Divisie at Helmond Sport in 2002. In 2005 he moved to FC Emmen where he scored 18 and 12 goals in his first two seasons. In the 2007/08 season he played at Emmen alongside his brother Danny.

On 16 June 2008, he was signed to a two year contract by Brisbane Roar (then operating as Queensland Roar) after impressing the Roar scouts with his technique at the end of May against Redlands United and a Gold Coast Select XI.