Website abgeordnetenwatch.de said 150 out of the 631 lawmakers in Germany's lower house, the Bundestag, earned money from other jobs.
In the nine months the parliament has sat, lawmakers made more than €6.6 million in additional earnings.
At least €2.1 million came from anonymous sources, the group said.
"In reality, the earnings are likely much higher," abgeordnetenwatch.de said in a blog post.
Conservatives top list
Most of the 13 MPs who made more than €100,000 since September are members of Chancellor Angela Merkel's conservative Christian Democrats (CDU) or their Bavarian sister party, the Christian Social Union (CSU).
Seven CDU and CSU MPs registered earnings in the top bracket, meaning they made more than €250,000 from one source.
CSU's deputy head Peter Gauweiler topped the list with additional income of close to €1 million. His earnings were from work from his legal practice.
"Since the bracket has no limited, actual incomes theoretically could come to €1 million or more," the transparency group said.
Gauweiler has missed 35 votes out of 45 where MPs names had to be registered in the first few months of parliament's term, according to Spiegel Online.
"Seen in light of his absences and the number of votes he missed, Mr Gauweiler's side earnings are indeed a problem," SPD lawmaker Ulrich Kelber tweeted on Sunday.
The list of the highest earners also featured Peer Steinbrück (pictured), the former finance minister who ran for chancellor for the centre-left Social Democrats (SPD) last year. Public pressure during the campaign forced Steinmeier to disclose how he had made almost €2 million during the last parliament. His earnings in this parliament have come from speeches, consultancy and writing work.
'Conflicts of interest'
The transparency group criticized the disclosure system as insufficient. It pointed to the case of farmer Johannes Röring, a CDU MP from Borken, North Rhine-Westphalia.
Röring received between €75,000 and €100,000 since the beginning of this year from a source which was only identified as "Contract partner 3."
"Is it a wealthy person? Or a company? Does politician Röring vote on bills that directly or indirectly affect his 'contract partner 3?'", abgeordnetenwatch.de said.
"Potential conflicts of interest are obvious," the group added.
In 197 cases affecting the 150 MPs earning extra money, it was unclear who paid the lawmakers.
Automatic pay increases
MPs in Germany currently make €8,667 a month. They also receive a tax-free €4,204 for district offices and employees.
Lawmakers can also claim up to €12,000 a year for office equipment.
Within Berlin, MPs also have free access to chauffeured cars. For travel on official duty outside the city, lawmakers can use trains for free or claim reimbursement for flight expenses.
Lawmakers with additional positions within parliament, such as the heads of parliamentary groups and committees and their respective deputies, can make thousands of euros in addition to their salary as an MP.
MPs' pay will rise to €9,082 a month in 2015 after parliament linked their salaries to increases of the average German gross wage.
Under the new law, pay increases will need to be approved once at the beginning of a term and then increase automatically until the next election.
President Joachim Gauck initially delayed the law because of constitutional concerns before signing it ten days after it was meant to go into effect.
Abgeordnetenwatch.de wants parliament to pass a strict transparency law, requiring MPs to disclose who paid them "from the first euro to the last cent."
The group cited disclosure rules for British MPs as an example.