During Prime Ministers Questions, Mr Johnson said people who self-isolate are "helping to protect all of us by slowing the spread of the virus" in announcing the sick pay changes as part of emergency coronavirus legislation.
He said: "If they stay at home and if we ask people to self-isolate, they may lose out financially.
"So, I can today announce that the Health Secretary will bring forward, as part of our emergency coronavirus legislation, measures to allow the payment of statutory sick pay from the very first day you are sick instead of four days under the current rules, and I think that's the right way forward.
"Nobody should be penalised for doing the right thing."
Labour leader Jeremy Corbyn said: "Is it true, as has been reported, that police forces are likely to become so overstretched by coronavirus that 999 response times will have to be extended and that even investigations into some murders will have to be halted as a result of this?"
Mr Johnson responded: "We are not at that stage or anything like that stage yet. He (Mr Corbyn) knows our police forces are well able to cope with all types of eventualities and they have long-standing arrangements to prepare them for such pressures."
Mr Corbyn said: "Under this Government, there are two million workers on low pay, many of them women in the care sector who are not eligible for statutory sick pay at the present time.
"The Prime Minister's statement just now is not clear whether it covers them or not and those on social security could face sanctions if they miss appointments and, therefore, they and their families will face terrible hardship.
"When the Prime Minister brings forward the emergency legislation, will he guarantee that workers' rights to sick pay from day one, as he's just indicated, will apply on statutory sick pay, will apply to all claimants and those people that are not currently eligible for statutory sick pay, therefore will have to make a terrible choice between health and hardship."
Mr Johnson replied: "Well (Mr Corbyn) is raising a very important point and, of course, we are very much aware of the issues faced by the self-employed and those on zero-hours contracts.
"I should stress that some of them will be entitled to statutory sick pay, others will be entitled to help through the existing systems such as Universal Credit and we are urgently looking at the application process to reflect on the advice on self-isolation."