The UN's political affairs chief, Jeffrey Feltman of the United States, will remain at this post, also for a year.
The two positions of under-secretary-general are among the most high-profile at the United Nations.
Lacroix, 56, will replace Herve Ladsous who steps down in March after six years in the top job.
Currently director for the United Nations at the French foreign ministry, Lacroix will take over UN peacekeeping as it struggles to overcome a scandal over mounting cases of sexual abuse by troops serving in peace missions.
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Guterres took over from Ban Ki-moon on January 1 with a pledge to reform the United Nations to improve its capacity to address global crises and prevent conflicts.
The UN chief said in a statement he is setting up an internal review team to look at the world body's peace and security strategy, functioning and architecture.
That team led by Tamrat Samuel of Eritrea will submit recommendations by June.
The push for reform comes amid criticism from some UN member states over the appointment of nationals from Security Council permanent members to top posts at the United Nations.
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