However, overall numbers of migrants have fallen by nearly 60% from the same period the previous year.
Over 100,000 migrants reached Europe from North Africa and the Middle East in the first half of 2017, according to data published by the International Organization of Migration.
The arrivals to Greece decreased to 12,191 this year, when compared to 161,232 last year, the IOM said.
Spain's increase is in stark contrast to the USA where President Trump's' administration has slowed illegal border crossings by more than 72 percent compared to 2016, according to a recent Washington Times article.
Italy soon replaced Greece as the centre of events, with the greatest number of migrants travelling from Libya across the Mediterranean.
Experts say it's hard to pinpoint why the water route to Spain via Morocco has become a preferred option for some migrants.
Officers were on high alert after 187 sub-Saharan migrants the previous day were able to cross the border by forming a compact group that ran through the Tarajal border crossing.
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"Given where the people are coming from, it's possible this might explain a shift away from Libya and the increased numbers in Spain", he said.
Even if the number of migrants to Spain continues to rise, it is unlikely to reach the record of some 40,000 arrivals the country had in 2006, according to the Spanish Red Cross.
Millman says that it is probable that Spain will take the lead from Greece for having received the most migrants by the end of 2017.
But crossings from Morocco to Spain - a distance of nine miles at its narrowest point - are growing rapidly. The number of sailings peaked in 2006 after Spain made loan and investment deals with several countries.
All of the migrants - about 700 - were repelled by the Moroccan and Spanish authorities.
More than 1,700 have been rescued from the water so far this year, with the month of June alone seeing more than 900.