In all, 8,300 more people left Wyoming than moved to the state over the a year ago but new births meant that the total loss of population was less than that.
Texas added more people in the past year than any other state, according to new estimates from the U.S. Census Bureau.
NY remains the nation's fourth most populous state with 19,849,399 residents.
Pennsylvania has seen a population growth of 18,400 people over the past few years.
The bureau added that the 33,700-person drop was a relatively small percentage change compared to Illinois' population of 12.8 million people.
Census officials confirmed IL lost the spot due to a consistent growth in Pennsylvania's population. The state is bedeviled by a mountain of debt and unmet pension obligations -Moody's Investor's Service estimates the state has $250 billion public pension debt.
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Previous year it was ranked 5th.
Meanwhile, all of Illinois' neighboring states experienced population growth in 2017.
All of Kentucky's population growth since 2010 has been in the population segment age 65 and older, which has grown by 23% since the last Decennial Census.
Rural counties and most downstate metro areas saw the most dramatic declines. Wyoming has roughly 579,000 people, while the "land of Lincoln" has over 6.6 million. Net global migration decreased 1.8% - the first drop since 2012 to 2013.
Colorado's population grew by 1.4 percent from July 1, 2016, to July 1, 2017, the seventh-fastest growth rate in the country, according to Census Bureau estimates released Wednesday.
From July 2016 to July 2017, almost 115,000 Illinoisans left for other states on net, according to the Census data.