Middlefield Collegiate Institute (MCI) is a semestered, public high school in the neighbourhood of East Milliken in the city of Markham, Ontario, Canada.

Middlefield Collegiate Institute was built in 1992 to feed the ever growing East Milliken community which is a suburban area.

In 2017, the new Aaniin Community Centre and Library was opened nearby and is frequented by students as a learning space and for social activities.

Middlefield was the only York Region school to win the soccer championship two years in a row, which started with the junior boys of 2003-2004.

Middlefield followed this up by winning the Ontario Bar Association Mock Trial Tournament and becoming Provincial Champions for the first time, defeating Cawthra Park Secondary School.

The percentage of eligible students who passed their Ontario Secondary School Literacy Test on the first attempt was 87.4%.