He accepted the offer of an alliance from Francois Bayrou, another centrist and devout Roman Catholic.
Recent polls suggest support for Mr Macron, tipped by many to win the election, has dropped slightly.
Two of far-right candidate Marine Le Pen's staff were detained on Wednesday as part of a financial inquiry.
Her chief of staff was placed under formal investigation over the alleged misuse of EU funds to pay parliamentary assistants, French media report. Her bodyguard was also questioned.
The National Front (FN) leader, who denies any wrongdoing, dismissed the investigation as a "political dirty trick".
She enjoys a solid lead in opinion polls for the first round in April but is seen as being easily defeated in the second.
Mr Bayrou has run three times for president, winning more than 18% of the vote in the 2007 election.
Announcing that he would not stand this year, he offered to join forces with Mr Macron to prevent "the failure of France".
Ms Le Pen, he said, posed a "major and immediate threat for our country and Europe".
Welcoming the news, Mr Macron, a free-market liberal, told Reuters news agency: "The alliance proposed by Francois Bayrou... fully matches our commitment to renewal and unity."
Analysts say Mr Bayrou's support base is likely to split now between Mr Macron and his conservative opponent Francois Fillon.
A new opinion poll indicates that Mr Macron and Mr Fillon are neck and neck in the race for the all-important second place in the first round - with both on about 19%.
According to the same poll, taken before the alliance was announced, Mr Bayrou would net 5.5% while Ms Le Pen would take 26%.
In the second round, Mr Macron would beat Ms Le Pen by 61% to 39%, while Mr Fillon would also defeat the far-right candidate, but by 57% to 43%.
Early on in the campaign, Mr Fillon was viewed as the clear favourite to reach the run-off along with Ms Le Pen and win, but a scandal over payments to his family dented his image in January.
He had talks with his fellow conservative former prime minister Alain Juppe, whom he beat in the Republican party primaries, on Wednesday, Le Figaro newspaper reports.
Ms Le Pen has been accused by the European Parliament of misusing a total of some 340,000 euros (Â£289,000; $359,000), AFP news agency reports.
It is alleged that she claimed a salary for her bodyguard Thierry Legier for a bogus job and paid her chief of staff, Catherine Griset, as her EU assistant in Brussels when Ms Griset was actually working for the party.
Mr Legier and Ms Griset were taken in for questioning on Wednesday.
"The French can tell the difference between genuine scandals and political dirty tricks," Ms Le Pen responded, drawing a distinction between her case and that of Mr Fillon.
This election promises to one of the most dramatic in decades in France, a core member of the EU with the world's sixth-largest economy.
Fillon apologises over family payments
Le Pen refuses to repay 'misused EU cash'
The leading centrist candidate in France's presidential election, Emmanuel Macron, has received a boost after allying with a veteran moderate.