Mrs May, one of several Conservatives touted as a future party leader, told delegates in Manchester that levels of immigration seen over the last 10 years were not in Britain’s national interest.
The government has made a commitment to cut net migration to tens of thousands, but is struggling to achieve this, with the latest figures showing that it reached a high of 330,000 people in the year to March.
Mrs May said mass immigration put a strain on public services, as well as pushing down wages.
“When immigration is too high, when the pace of change is too fast, it’s impossible to build a cohesive society. It’s difficult for schools and hospitals and core infrastructure like housing and transport to cope,” she said.
“And we know that for people in low-paid jobs, wages are forced down even further, while some people are forced out of work altogether.
“But even if we could manage all the consequences of mass immigration, Britain does not need net migration in the hundreds of thousands every year.”
In the midst of the migrant crisis, which has seen big movements of people from the Middle East and Africa to Europe, Mrs May said that the “desire for a better life is perfectly understandable”.
But she added: “There are millions of people in poorer countries who would love to live in Britain, and there is a limit to the amount of immigration any country can and should take.
“While we must fulfil our moral duty to help people in desperate need, we must also have an immigration system that allows us to control who comes to our country.”
Prime Minister David Cameron has said he will step aside after two terms in Downing Street, and told Sky News he “won’t be tempted” to prolong his stay.
He said: “I’ve also got a talented team behind me and after I’ve done the two terms, the 10 years, I’m sure there’ll be many talented people who put their name forward and frankly I’m proud of the fact that they are increasingly being noticed as a talented bunch.”
Mr Cameron praised Mrs May as an “excellent” home secretary.