Well it depends on who you ask. Last week Japan made it legal by legislating to bring bitcoin exchanges under KYC/AML laws and at that same time recognise bitcoin as a form of payment.
In most countries it is not illegal but then again it has not been recognised as legal tender so there is a risk that governments may introduce regulations that seek to curtail or eliminate the use of bitcoin or other cryptos in their jurisdictions. I suspect that in jurisdictions where cryptocurrencies start to threaten the usefulness of the national fiat currency, governments are likely to be more aggressive in seeking to curtail the growth of cryptos.
One example case in point is India. It is not illegal in India but the Government has been making noises about regulating bitcoin. It is not clear how this will pan out but all eyes are on a report/meeting scheduled for 20 April, which is expected to set out the Government’s position (at least for now) with bitcoin and cryptos.