Cecilius Calvert, 2nd Baron Baltimore, 1st Proprietor Governor of Maryland, 9th Proprietor Governor of Newfoundland (August 8, 1605 – November 30, 1675), usually called Cecil, was an English coloniser who was the first proprietor of the Maryland colony. He received the proprietorship that was intended for his father, George Calvert, the 1st Lord Baltimore, who died shortly before it was granted. Cecil established Maryland from his home in England, and as a Roman Catholic continued the legacy of his father by promoting religious tolerance in the colony. Forming it based on the ideas of freedom of religion and separation of church and state, Maryland became known as a haven for Roman Catholics in the New World. Cecil governed Maryland for forty-two years. He also served as Governor of Newfoundland. He died in England on November 30 1675, aged 70 years.

Cecilius Calvert, whose first name was sometimes spelled Cæcilius, or Caecilius, was born on August 8, 1605, in Kent, England. He married Anne Arundell, daughter of the 1st Baron Arundell of Wardour, in 1627 or 1628. They had nine children. Out of the nine children, only three, including Charles Calvert, 3rd Baron Baltimore, made it to adulthood.

He entered Trinity College at the University of Oxford in 1621. On August 8, 1633, he was admitted to Gray's Inn.

Calvert received a charter from Charles I of England for the new colony of Maryland, named for the Queen Consort Henrietta Maria, shortly after the death of his father, the 1st Baron Baltimore, who had long pursued a colony in the mid-Atlantic to serve as a refuge for English Catholics. The original grant would have included the western shore of the Chesapeake Bay as far south as the Potomac River and the entirety of the eastern shore. When it was realized that settlers from Virginia had already crossed the bay to begin settling the southern tip of the eastern shore, the grant was revised to include the eastern shore only as far south as a line drawn east from the Potomac River. Once that alteration was made, the final charter was confirmed on June 20, 1632. Baltimore's fee for the charter, which was legally a rental of the land from the King, was one-fifth of all gold and silver found and the delivery of two Native American arrows to the royal castle at Windsor every Easter. It established Maryland as a palatinate, gave to Baltimore and his descendants rights nearly equal to those of an independent state, including the rights to wage war, collect taxes, and establish a colonial nobility. If at any point there was a question as to the rights contained within the charter, the charter would be interpreted in favor of the proprietor. The new charter was opposed in England by supporters of Virginia, who had little interest in having a competing colony to the north. Rather than going to the colony himself Baltimore stayed behind in England to deal with this threat and sent his younger brother Leonard in his stead.

While the expedition was being prepared, Baltimore was kept busy in England defending the charter from former members of the Virginia Company who were trying to regain their charter, including the entirety of the Maryland colony which had previously been a part of Virginia. They had informally sought to thwart Baltimore's efforts for years, but their first formal complaint was lodged with the Lords of Foreign Plantations in July, 1633. The complaint claimed that Maryland had not truly been unsettled, as claimed in its charter, because of the presence of a trading station run by William Claiborne on Kent Island. It also claimed that the charter was so broad as to constitute a violation of the liberties of the colony's citizens, although at this point there were not a great many Marylanders to speak of.

The first expedition consisted of two ships that had formerly belonged to Baltimore's father, the Ark and the Dove. They departed Gravesend with 128 settlers on board and, after being chased down and brought back by the British navy so that the settlers could take an oath of allegiance to the King as required by law, sailed in October of 1632 for the Isle of Wight to pick up more settlers. At the Isle of Wight they boarded two Jesuit priests and nearly two hundred more settlers before setting out across the Atlantic. Since he could not lead the expedition himself, Baltimore sent detailed instructions for the governance of the colony, including commands that his brothers seek any information about those who had tried to thwart the colony and make contact with Claiborne to determine his intentions for the trading station on Kent Island. The instructions also emphasized the importance of religious toleration among the colonists, who were nearly equal parts Catholic and Protestant. With these last instructions, the expedition crossed the Atlantic and founded the first settlement at St. Mary's in 1634 on land purchased from the native Yaocomico. Stranded in England, Baltimore could do little to help the young colony through its tribulations, which included an ongoing feud with Claiborne that led to a series of naval skirmishes.

Lord Baltimore attempted to maintain an active involvement in the governance of the colony, though he never visited it. During this long tenure, he governed through deputies, the last being his only son Charles. Calvert's younger brother, Leonard Calvert (1606 – 1647), was the first Governor of the Province of Maryland. He was the second son of George Calvert, 1st Baron Baltimore, the first proprietor of the Province of Maryland. His elder brother Cecilius appointed Leonard governor in his absence.

In 1649 Maryland passed the Maryland Toleration Act, also known as the Act Concerning Religion, a law mandating religious tolerance for trinitarian Christians. Passed on September 21, 1649 by the assembly of the Maryland colony, it was the first law requiring religious tolerance in the British North American colonies. The Calvert family sought enactment of the law to protect Catholic settlers and those of other religions that did not conform to the dominant Anglicanism of Britain and her colonies.

The enterprise took place in the context of serious unrest in England. In 1629, King Charles had dissolved Parliament and governed for the next eleven years without input from a representative body. The Church of England, led by the Star Chamber, intensified its campaign against both Puritans and Catholics. The former were able to flee England to their New England colony, but for Catholics, Maryland was their sole place of refuge from persecution. Lord Baltimore, a Roman Catholic, struggled to maintain possession of Maryland during the English Civil War by trying to convince Parliament of his loyalty by appointing a Protestant, William Stone, as his governor. Baltimore lost control of the colony for a brief period, however, due to Puritan pressure during the rule of Oliver Cromwell. He regained the colony in 1657, after signing a treaty with Virginia Governor Richard Bennett. The primarily Puritan assembly retained powers in Maryland until April 27, 1658, when proprietorship was restored to Lord Baltimore, religious freedom was ensured, and an agreement of general amnesty was entered into. Thus, in the end, Lord Baltimore not only retained his lands and powers, but was able to avoid the grisly fate of many of his contemporaries in England during this time. The proprietor appointed Josias Fendall to succeed Stone as governor for Fendall's loyalty during the Battle of the Severn.

Governor Fendall soon had a falling out with Lord Baltimore and led a bloodless revolution in 1659 whereby he and William Fuller reorganized Maryland's government to resemble the Commonwealth's. However, the Restoration of Charles II in 1660 forced Fendall into exile and restored the proprietorship.

Lord Baltimore's family also had title to Ferryland and the Province of Avalon in Newfoundland and he administered the colony between 1629 and 1632 when he left for Maryland. In 1637, however, Sir David Kirke acquired a charter giving him title to the entire island of Newfoundland superseding the charter granted to his father, the 1st Baron. Baltimore fought against the new charter and, in 1661, gained official recognition of the old Charter of Avalon but never attempted to retake the colony.

Cecilius Calvert died in Middlesex, England on November 30, 1675. He was succeeded to the Baronetcy of Baltimore and to his other titles by his son and heir, Charles Calvert, 3rd Baron Baltimore.

There are several locations in Maryland named after the Barons Baltimore, including Baltimore County, Baltimore City, Calvert County, Cecil County, Charles County, Frederick County, Leonardtown, St. Leonard and Calvert Cliffs. Anne Arundell's name survives in that of Anne Arundel County, Maryland. His name survives in that of Cecil County, Maryland, Cecil Avenue and Calvert Street in Baltimore City, and Calvert street in Washington, DC. Harford County is named for Henry Harford, the illegitimate son of Frederick Calvert, 6th Baron Baltimore, who while not able to inherit the peerage, did inherit the Lord Proprietorship, only to lose it during the Revolution. There is also a Charles Street in Baltimore. The main downtown street in Cumberland, Maryland, is named Baltimore Street. On the Avalon Peninsula in Newfoundland, there is a settlement named Calvert, and in nearby Ferryland there is a Baltimore School.

A life-sized statue of Cecilius Calvert is located in front of the Clarence M. Mitchell, Jr. Court House in Baltimore, Maryland.